development of a lesson in physics 8 cells.
goal: to study the concept of light and light sources.
educational: introduce students to natural and artificial light sources, explain the law of rectilinear propagation of light, consider the nature of solar and lunar eclipses, consolidate the ability to construct the path of rays during the formation of shadow and penumbra; to continue work on the formation of experimental research skills.
educational: to form cognitive interest; develop the ability to work in a group and respect the opinions of classmates; contribute to the formation of a scientific worldview,
developing: develop attention, imagination, observation, logical and critical thinking. promote the development of cognitive interests, intellectual and creative abilities during the lesson and when doing homework using various sources of information and modern information technologies; create conditions for the development of creative and research skills, form the ability to highlight the main thing, compare, draw conclusions; develop speech, improve intellectual abilities
Forms of organizing the work of children:
Individual, frontal, group,
Forms of study: visual, practical (exercises); frontal work, independent work, conversation on questions, individual tasks.
Type and type of lesson: learning new material
Teaching methods:
heuristic method
research,
explanatory-reproductive,
encouraging,
Equipment: teacher's computer or laptop, multimedia projector, screen, light sources, bodies of different sizes.
Results of the training session:
subject- generalize and systematize students' knowledge about light sources, the laws of light propagation, find out the meaning of light in human life; to form the ability to explain the reasons for the formation of shadow and penumbra, solar and lunar eclipses; to form the ability to conduct experiments, to explain the results of research.
Metasubject- to develop the creative abilities of students in the course of performing creative tasks; develop skills in the use of information technology and various sources of information to solve cognitive problems; broaden the horizons of students, show the application of theoretical knowledge in practice; develop the ability to analyze and creative activity, the ability to think logically; develop interest and logical thinking by solving educational problems, explaining interesting facts.
Personal- the formation of an active life position, a sense of collectivism and mutual assistance, the responsibility of each for the final results; education of independence, diligence, perseverance in achieving the goal.
during the classes:
1. Org moment. Checking the readiness for the lesson, the mood for work.
Hello guys, check the readiness for the lesson (accessories, textbook, notebook)
2. Preparation for the perception of new material.
Guys! We continue to get acquainted with new concepts in physics, to discover something new and interesting. And how much more unexplored around? Interest in everything unknown arises when a person works on his own.
Even if you go out not into the white light, but into the field beyond the outskirts,
When you follow someone in the trail, the road will not be remembered.
For that, wherever you go, and for what kind of mud
The road that he himself was looking for will never be forgotten!
So, at the beginning, I suggest that you determine the topic of the lesson (working with cards) Guys, you have tasks in which the phone number is encrypted, by which you can find out the topic of the lesson, but at the beginning you need to guess the phone number.
Questions:
1. How many planets in our solar system does the Sun illuminate? (8)
2. Every year in the morning
He enters through the window to us.
If he's already in
5. Lodygin .................... invented the incandescent electric light bulb
6. The day is gone, the distance has faded,
The birds have stopped singing
What flickers in the sky? (9-stars, 2-bulb, 8-fireflies)
7. Splashed a little milk
Someone star track
She is in the velvet sky
Dissolved, barely visible.
I look up - I can’t fall asleep!
8. Suddenly lit up on a blade of grass
A real flame.
It's with a light on the back
sparkles, blinks,
10. The head burns with fire,
The body melts and burns.
I want to be useful
There is no lamp - I will shine.
(9-Candle, 1-flashlight, 7-phone)
11. Servants of His Majesty
Luminous Electricity.
In bows stand along the road
And shine under the feet of passers-by.
(8-cars, 2-electricians, 4- Lanterns.)
Well done, guessed the phone number, and now let's call the number and find out what to do next. (calling)
Question on the phone: Guess what unites the questions in the card, is this the topic of the lesson? (light) Let's write down the topic of the lesson: "Light. Sources of light. Spread of Light"
2. Explanation of new material
Task number 1: Guys, I propose to study the list of keywords of the new topic and individually fill in the columns of the following table: (the children have a table on the desk)
| topic keywords | I know | Do not know |
| Light source | ||
| natural light source | ||
| penumbra | ||
| artificial light source | ||
| point light source | ||
The interesting thing is that you guys have just started learning a new topic, but have already shown knowledge of some concepts.
What is the purpose of the lesson?
what is light, what sources of light exist; what sources are point; how light propagates in a homogeneous medium;
Let's close our eyes for a moment and imagine “life in darkness”!!! Do you see the beauty of our world? What are your feelings? The world has become paler for us... It is hard to imagine life without light. After all, all living things exist and develop under the influence of light and heat. What helps us to understand the world around us? Light... Its significance in our life is very great. Today we will talk about one of the areas of physics where light phenomena are studied. You will learn: what is light, what bodies are sources of light, what are the laws of light propagation.
Human activity in the initial periods of its existence - the extraction of food, protection from enemies - was dependent on light. Light, due to the fact that the human eye is able to perceive it, is the most important means of understanding nature. When dawn comes after a long darkness, everything seems to come to life: both trees and water. And the sky. And birds. Vision allows us to learn more about the world around us than all the other senses combined. The study of light phenomena made it possible to create such instruments, with the help of which they determined the location and movement, and even the composition of celestial bodies. And also managed to look inside the bodies. Using a microscope, we examined the composition of the cell, studied the structure of bacteria, blood cells.
Light is needed everywhere: Traffic safety on the roads is associated with the use of headlights, street lighting; in military equipment, flares and searchlights are used. Light increases the body's resistance to disease, improves the health and mood of a person. Workplace lighting enhances productivity.
So what is light? Let's find the definition in the textbook(p. 147) write it down. light is radiation, but only that part of it that is perceived by the eye;
The second question we asked what are light sources?(we will find the exact definition in the textbook p. 147) Sources - bodies capable of emitting light.
We see not only sources of light, but also bodies that are not sources of light - a book, a school desk, houses, etc.
We see these objects only when they are illuminated.
The radiation coming from the light source, hitting the object, changes its direction and enters the eye.
what we wanted to know about light sources? (their types)
So, for a better understanding, I will now demonstrate to you the sources available in the physics classroom (demonstrates a burning candle, an electric incandescent lamp, a fluorescent lamp, a laser, a phosphorescent screen, a source of ultraviolet radiation). The sun, fire, lightning, a hot piece of metal are examples of thermal light sources that glow because they have a high temperature. Amazing heat sources are stars - huge celestial bodies. Many of them are much larger than the Sun. Since the stars are very far from us, they are visible in the sky as luminous dots. Such objects are referred to as point sources of light.
There are substances that themselves begin to glow after illumination. They are called luminescent substances. Translated from Latin, "luminescence" means "Glow". Sometimes a mechanical shock can cause luminescence. If specially made glass tubes filled with various rarefied gases are connected to a high voltage current source, then an electric current arises in the gases - a discharge. Such tubes are called gas-discharge tubes. The color of the glow in them depends on the nature of the gas and the degree of its discharge.
The teacher gives precise definitions of concepts: light sources are bodies that create light (optical) radiation. We see light sources because the radiation they create hits our eyes. The general principle on which the action of all light sources is based is the conversion of any energy into light energy.
physical minute
if you hear the name of a natural light source, raise your right hand, artificial - left, thermal - turn your head to the right, point - turn your head to the left
Task 2
Place the candle and screen with a vertical slot on a piece of white paper. Light a candle and watch the strip of light behind the screen.
Mark with a pencil on paper point A near the candle, point B opposite the gap and point C on the beam of light behind the screen. Remove the screen and use a ruler to draw a straight line AB connecting the candle and the gap in the screen. Then draw a straight line BC along the strip of light behind the screen. Make sure that line BC is an extension of line AB. Make a conclusion.
Task 3
Leave the burning candle at point A, and place the screen at point C. Place an opaque cylinder at point B between the light source and the screen. Turn on the lamp and observe the propagation of light behind the cylinder. Make a conclusion.
Move the cylinder close to the screen and illuminate it with light. As you move the light source closer and closer to the cylinder, watch the image of the cylinder change on the screen. Analyze the result.
Students' responses are written on the board.
Light travels in a straight line.
The brightness of a light beam depends on the distance to the source.
The divergence of the beam depends on the distance to the source.
The screen is a barrier to light.
The size of the shadow depends on the distance between the subject and the light source.
The shape of the shadow depends on the location of the subject and the light source.
All the conclusions expressed by you are correct, but I want to draw attention to only one of them. It is one of the four basic laws of light propagation.
Light in a homogeneous medium from a source propagates in a straight line and in all directions. The line along which light travels is called a light beam.. There are several experimental proofs of this law. The screen is illuminated by an illuminator. An opaque disc is placed in the path of light propagation. A clear image of the shadow appears on the screen. The area of space that is not hit by light from a light source is called a shadow. The experiment is repeated, but the light source is first slowly brought closer to the opaque disk, and then removed from it. Students' attention is drawn to the size and shape of the shadow. The size of the shadow depends on the distance to the light source. As the light source approaches, the size of the shadow increases. As the distance between the source and the object increases, the size of the shadow decreases to the size of the object. An opaque disk from the previous experiment is illuminated by two adjacent illuminators. The screen shows an area where no light from any of the illuminators hits, and the pale shadows of the disk. Partially illuminated space is called penumbra. The globe of the Earth is illuminated by a projection device. A white ball imitating the Moon is moved around the globe on a high thin stand. When the ball is between the illuminator and the globe, its shadow falls on the surface of the globe. In that place of the Earth, where the shadow of the Moon falls, there is a solar eclipse. When the ball, when moving around the globe, enters the shadow of the globe, it ceases to be illuminated by the light source. If the Moon, during its revolution around the Earth, falls into the shadow cast by the Earth, then a lunar eclipse is observed. When the globe of the Earth is illuminated by two illuminators, it can be seen that the ball imitating the Moon casts a shadow and penumbra. If people on the surface of the Earth are in the shadow region, then they observe a total solar eclipse, and when they are in the penumbra region, they observe a partial solar eclipse.
physical minute « Hole in palms»
Doing practical work part 2
Formation of shadow and penumbra from two light sources
Observation of rectilinear propagation of light. Formation of shade and penumbra.
Using two lamps, a current source, a key, conductors, a variable rheostat, assemble an electrical circuit. Opaque body, screen.
Place the lamps at a distance of 1-2 cm from each other.
Place the screen at a distance of 20-25 cm from the lamps.
Close the chain.
Place an opaque object between the lamps and the screen.
Cover one lamp with your hand. Mark the shadow area on the screen.
Cover another lamp with your hand. Mark the shadow area on the screen.
Get the shadow area from the two lamps.
Achieve, by changing the position of the subject, a partial overlay of shadows on each other.
Draw a zone of shadow and penumbra on the screen.
Make a conclusion based on the results of the study.
III. Problem solving:
A person reading a book does not care whether the source of light is to the right or to the left of him. Why is it so important when writing that the light falls from the left?
The sun is shining and the moon is shining .(explain the meaning of this proverb)
Determine the length of the shadow from a person whose height is 160 cm, if the length of the shadow from a meter ruler is 1.5 meters?
IV. Interesting Facts:
Interestingly, a sea worm saves a life. When the crab bites into it, the back of the worm flares up brightly. The crab rushes to it, the injured worm hides, and after a while a new one grows in place of the missing part.
In Brazil and Uruguay, there are reddish-brown fireflies with rows of bright green lights along the body and a bright red “bulb” on the head. There are cases when these natural lamps - inhabitants of the jungle - saved people's lives: during the Spanish-American War, doctors operated on the wounded by the light of fireflies poured into a bottle.
In the 18th century, the British landed on the coast of Cuba, and at night they saw tirades of lights in the forest. They thought that there were too many islanders and retreated, but in fact they were fireflies.
The direction to the north in the northern hemisphere is determined by standing at noon with your back to the Sun. The shadow cast by a man, like an arrow, will point north. In the southern hemisphere, the shadow will point south.
The Hamburg alchemist Brand spent his whole life looking for the secret of obtaining a “philosopher's stone” that would turn everything into gold. Once he poured urine into a vessel and began to heat it up. When the liquid evaporated, a black precipitate remained at the bottom. Brand decided to ignite it on fire. A white substance similar to wax began to accumulate on the walls of the vessel. It glowed! The alchemist thought he had fulfilled his dream. In fact, he received a previously unknown chemical element - phosphorus. .(bearing light)
Students answer the questions:
Teacher: Kozma Prutkov has an aphorism: “If you are asked: what is more useful, the Sun or the moon? - answer: a month. For the Sun shines during the day, when it is already light, and the moon at night.” Is Kozma Prutkov right? Why?
Teacher: What are the sources of light that you had to use when reading?
Teacher: A heated iron and a burning candle are sources of radiation. What is the difference between the radiation produced by these devices?
Teacher: From the ancient Greek legend about Perseus: “Not further than the flight of an arrow was a monster when Perseus flew high into the air. His shadow fell into the sea, and with fury the monster rushed at the shadow of the hero. Perseus boldly rushed from a height to the monster and deeply plunged a curved sword into his back.
Teacher: What is a shadow and what physical law can explain its formation?
Teacher: What actually determines the apparent shape of the moon?
Teacher: We solve quality problems.
1. How can light sources be positioned so that during the operation the shadow from the surgeon's hands does not cover the operation site?
Answer: Arrange several lamps overhead
2. Why don't objects cast shadows on a cloudy day?
Answer: Objects are illuminated by diffused light, the illumination from all sides is the same.
3. Is it possible to observe solar and lunar eclipses from any point on the Earth's surface?
Answer: Lunar yes. Solar no.
4. Can a cyclist overtake his own shadow?
Answer: Yes, if a shadow is formed on a wall parallel to which the cyclist is moving, and the light source is moving faster than the cyclist in the same direction.
5. How does the size of the penumbra depend on the size of the light source?
Answer: The larger the source, the greater the penumbra.
6. Under what condition should the body give a sharp shadow on the screen without penumbra?
Answer: When the size of the light source is much smaller than the size of the body.
Test:
1. There are light sources
A. ... only natural.
B. ...only artificial.
B. ... natural and artificial
2. What light source is called a point?
A. Luminous body of small size. B. a source whose dimensions are much smaller than the distance to it. B. A very weakly luminous body.
3. How does light propagate in a homogeneous medium?
A. straightforward
B. curvilinear.
B. Along any line connecting the source and the subject.
4. How light sources are divided
A. Point and extended
B. mechanical
B. thermal
5. What is the source of visible light?
A) a heated electric kettle
B) TV antenna.
B) Welding arc
6. Among the listed sources does not emit light?
A) Bonfire;
B) Radiator;
B) the sun.
7. What does the shadow represent?
A) A region of space where, due to rectilinear propagation, light does not fall.
B). Dark place behind the subject
C) a place that a person cannot see
8. What is penumbra? What should be the source.
A) The place where the light partially hits. Extended.
B) A place where there is light, but it is not enough.
C) A region of space where there is both shadow and light. Dotted.
9. What line is called a light beam?
A) a line emanating from a light source
B A line along which energy propagates from a light source.
C) The line along which light from a source enters the eye.
Teacher: Answers are offered to you, and you yourself can evaluate your work:
0 errors - 5
1-2 mistakes - 4
3-4 mistakes - 3
5-6 mistakes - 2
Teacher: Today in the lesson we got acquainted with light sources, learned that in a homogeneous medium, light propagates in a straight line. Evidence: the formation of shadows and penumbra, solar and lunar eclipses.
Teacher: Have we reached the goal that we set at the beginning of the lesson?
Pupils: Fixed the studied material; tested the acquired knowledge.
Experiment: Take a meter stick and measure the size of its shadow on the street. Then determine the actual height of trees, houses. pillars, measuring their shadows.
Your mood at the end of the lesson and reflect it on the smiley.
Teacher: Guys! In conclusion, I want to say. The physicist sees what everyone sees: objects and phenomena. He, like everyone else, admires the beauty and grandeur of the world, but behind this beauty accessible to everyone, he discovers another beauty of patterns in an endless variety of things and events.
consolidation
Choose the correct answer for each question (one question can have more than one answer). For example, if you consider the answers to the first question numbered 3 and 5 to be correct, then write it down like this: 1 (3.5), if there is no correct answer, then 1 (-).
| 1. Section of science that studies light and light phenomena - | 1. light fell from the left so that no shadow was formed |
| 2. Name natural light sources | 2.when heated, the process of liquid evaporation occurs |
| 3. Name artificial light sources | 3. Due to the illumination by the light source. The radiation coming from light sources, hitting the surface of the object, changes its direction and enters the eyes. |
| 4. according to sanitary standards, students in classrooms should sit so that the light falls on the left | 4. magnifier, telescope, camera, periscope |
| 5. The arc in electric welding is | 5.visible light source |
| 6.Based on the study of light phenomena, devices have been created: | 6.computer screen, el. light bulb, flashlight |
| 7. Under the influence of sunlight, fruits dry out, because | 7.firefly, rotten, lightning |
| 8. We see bodies that are not a source of light ... | 8.called optics |
| 9.because we look carefully |
|
| 10.artificial source |
|
| 11. tiles, boiler, telegraph |
|
| 12. candle flame, electric arc |
Reflection. Sincwine.
The word "cinquain" comes from a French word that means "five". Thus, cinquain is a poem consisting of five lines:
1 - one word, usually a noun, reflecting the main idea;
2 - two words, adjectives describing the main idea;
3 - three words, verbs that describe actions within the framework of the topic;
4 - a phrase of several words, showing the attitude to the topic;
5 - a word or several words associated with the first, reflecting the essence of the topic.
Questions:
1. How many planets in our solar system does the Sun illuminate?
2. Every year in the morning
He enters through the window to us.
If he's already in
So the day has come. (answers: 2 - wind, 9 - light, 3 - noise)
3. A pear is hanging - you can’t eat it? (0-light bulb, 2- Christmas toy, 6- drawing)
4. He eats everything, but is he afraid of water? (0-cat, 5-fire, 9-child)
5. Lodygin .. (numeral) .............. invented the incandescent electric light bulb
6. The day is gone, the distance has faded,
The birds have stopped singing
They lay down until dawn in the nests ...
What flickers in the sky?
(9-stars, 2-bulb, 8-fireflies)
7. Splashed a little milk
Someone star track
She is in the velvet sky
Dissolved, barely visible.
I look up - I can’t fall asleep!
What is in the sky? (1-moon, 3-comet, 2-milky way)
8. Suddenly lit up on a blade of grass
A real flame.
It's with a light on the back
Sat on the grass ... (7-firefly, 4-bug, 3-mosquito)
sparkles, blinks,
Shoots curved arrows. (1 - sniper, 2 - lightning, 7 - Zeus)
10. The head burns with fire,
The body melts and burns.
I want to be useful
There is no lamp - I will shine. (9-Candle, 1-flashlight, 7-phone)
11. Servants of His Majesty
Luminous Electricity.
In bows stand along the road
And shine under the feet of passers-by. (8-cars, 2-electrics, 4-lanterns.)
| topic keywords | I know | Do not know |
| Light source | ||
| natural light source | ||
| penumbra | ||
| artificial light source | ||
| point light source | ||
| topic keywords | I know | Do not know |
| Light source | ||
| natural light source | ||
| penumbra | ||
| artificial light source | ||
| point light source | ||
| topic keywords | I know | Do not know |
| Light source | ||
| natural light source | ||
| penumbra | ||
| artificial light source | ||
| point light source | ||
| topic keywords | I know | Do not know |
| Light source | ||
| natural light source | ||
| penumbra | ||
| artificial light source | ||
| point light source | ||
Exercise 1
Task 2
Exercise 1
Place the screen with the vertical slot on a sheet of white paper. turn on the flashlight of the phone and watch the strip of light behind the screen.
Draw a conclusion about how light travels (in a straight line, along a curve)
Task 2
1. Burning candle, place the screen opposite each other. Place an opaque cylinder between the light source and the screen. Move the cylinder close to the screen and move it away from the screen, watch the cylinder image change on the screen.
2. Moving away and bringing the light source closer to the cylinder, observe the change in the image of the cylinder on the screen. Analyze the result. Make a conclusion.
Exercise 1
Place the screen with the vertical slot on a sheet of white paper. turn on the flashlight of the phone and watch the strip of light behind the screen.
Draw a conclusion about how light travels (in a straight line, along a curve)
Task 2
1. Burning candle, place the screen opposite each other. Place an opaque cylinder between the light source and the screen. Move the cylinder close to the screen and move it away from the screen, watch the cylinder image change on the screen.
2. Moving away and bringing the light source closer to the cylinder, observe the change in the image of the cylinder on the screen. Analyze the result. Make a conclusion.
Exercise 1
Place the screen with the vertical slot on a sheet of white paper. turn on the flashlight of the phone and watch the strip of light behind the screen.
Draw a conclusion about how light travels (in a straight line, along a curve)
Task 2
1. Burning candle, place the screen opposite each other. Place an opaque cylinder between the light source and the screen. Move the cylinder close to the screen and move it away from the screen, watch the cylinder image change on the screen.
2. Moving away and bringing the light source closer to the cylinder, observe the change in the image of the cylinder on the screen. Analyze the result. Make a conclusion.
Kaminsky A.M. Original quality tasks. Optics // Physics: problems of laying out. - 2000. - No. 1. - S. 19-25.
1. Fish in Central America Anabbepssees well in both environments. She swims at the very surface of the water, so that her eyes protrude from the water. Why is this possible?
This fish has two retinas, and the lens is egg-shaped. In that part of the eye that is immersed in water, the lens area has a large curvature.
2. How do "one-way mirrors" work, allowing you to see through them in one direction while reflecting light in the other?
One side is brighter than the other. The weak image of the observer is lost against the background of a powerful light flux reflected by the mirror.
3. Why should you not water the leaves of garden plants on a sunny day?
The droplets focus sunlight onto the surface of the leaf and it chars.
4. Why do cat eyes glow in the dark when a flashlight is directed at them?
In carnivores, the eyes reflect light. Their eyes are a system of lenses and a curved mirror that reflects light onto a source.
5. How far away from us does a rainbow form, i.e. at what distance are those drops of water due to which it arises.
For a rainbow, only the angle between the incident sunbeam and the observer's line of sight matters. Drops can be located at a distance of several meters to several kilometers.
6. Sometimes circles are observed around the Sun or Moon (small Halo). It is usually located at an angular distance of 22° and is colored red on the inside and white or blue on the outside. Why does it arise? Is it true that Halo is considered a harbinger of rain?
The Small Halo is due to the refraction of light in incident ice crystals. The principal axes of the crystals on which the Halo is formed are randomly oriented in a plane perpendicular to the incident light beam. Therefore, at any point at an angle of 22 ° there are crystals that are oriented so that they give a bright light. Blue rays are refracted the most, so the outer side is painted in this color.
7. Tradition says that the Vikings owned a magical "sun stone", with which they could find the Sun behind the clouds and even beyond the horizon (in high latitudes, the Sun at noon can be below the horizon). What crystal and what phenomenon did the Vikings use?
It is believed that the Vikings used corderite crystals. If the incident light is polarized along one of the two axes of this crystal, then the crystal appears transparent. If the light is polarized along the other axis, then the crystal appears dark blue. By turning it and watching the color change, the Vikings could determine the direction of light polarization. With experience, you can find the direction of the Sun, even if it is beyond the horizon, since the light scattered by the sky is polarized.
8. Why isn't all of the sky the same hue, but some of it is a brighter blue?
Sunlight is scattered by air molecules, and light with a shorter wavelength is scattered more. Therefore, when the Sun is close to the horizon, the sky above the observer is mostly blue. Blue sky in the distance more than 90° from the Sun is weaker, as the sky is illuminated light that has traveled a longer distance in the atmosphere and lost its blue component.
9. Why ordinary clouds inmostly white, but storm clouds are black?
The size of water droplets in a cloud is much larger than air molecules, so the light from them is not scattered, but reflected. At the same time, it does not decompose into components, but remains white. Very dense thunderclouds either do not transmit light at all or reflect it upwards.
10. Sometimes there are mother-of-pearl clouds that have very beautiful tones. They are rare and are observed only at high latitudes. After sunset, they are so bright that the light from them colors the snow. What are the characteristics of these clouds?
Mother-of-pearl clouds are located at a very high altitude and consist of droplets whose radii (0.1-3 microns) are close to the wavelength of visible light. On these drops, light is diffracted, which depends on both the droplet radius and the wavelength.
11. Why do the beams of searchlights, which were used during the war to detect aircraft, cut off so abruptly in the air?
The beam is weakened not only due to divergence, but also due to atmospheric scattering. Therefore, its intensity falls off exponentially and ends quite abruptly.
12. On a moonless night, the zodiacal light and counter-radiance are visible in the sky. The zodiacal light is a hazy triangle that can be observed in the west for several hours after sunset or in the east before sunrise. Counter-radiance is a rather weak glow that occurs in the opposite direction to the sun. How to explain such glows?
These glows are associated with the scattering of light by cosmic dust coming from the asteroid belt. The zodiacal light is due to the dust inside the Earth's orbit. Counter-radiance is light scattered by dust outside of Earth's orbit.
13. If you stand on the mountain with your back to the sun and look into the thick fog spreading in front of you, you can see a rainbow border (or a closed ring) around the shadow of the head. Why does a halo occur and how are the colors arranged in it?
The halo arises due to the reverse (toward the source) scattering of light by water drops, the size of which is commensurate with the wavelength of the light. The returning light enters the drop from the side and exits from the side (but from the other side), having undergone reflection inside the drop, and also rounding it along the surface (diffraction). The backscatter angle depends on the wavelength, so colored rings are formed; since the angle also depends on the size of the droplets, the rings appear only when the droplets do not differ greatly in size.
14. The sun or moon are sometimes surrounded by a bright stripe - a crown. Usually the crown is a white stripe, but sometimes the white is followed by blue, then green and red. What caused it?
The crowns around the Sun and Moon are due to the diffraction of light by water droplets. Light rays coming from different sides of the drop interfere with each other. In this case, light and dark rings appear. If the drops are the same size, then it is possible to distinguish rings of different colors.
15. During a night walk, you can often see a rainbow halo around street lamps even in clear weather. Why?
The crowns around the lanterns are explained by the diffraction of light by obstacles commensurate with the wavelength of the light. But in this case, the particles are inside the eye itself. These are the radial fibers of the lens of the lens or particles of mucus on the surface of the cornea.
16. Why can you see your shadow in cloudy water, but not in clear water?
In order to see your own shadow on muddy water, you must be able to pick out the light reflected from the surface of the water. In clear water, this relatively weak light is lost against the light reflected from the bottom. When the water is cloudy, the light reflected from the bottom is greatly attenuated or absorbed, so shadows form.
17. If you bring the thumb and forefinger close together, then a dark line appears between them. Why?
A dark line is a set of dark fringes of an interference pattern that occurs when light is diffracted by a gap between the fingers.
18. What are those little blurry dots that sometimes increase and sometimes decrease in front of your eyes?
Spotting in the eye is an interference pattern caused by light diffraction from round blood cells floating just in front of the retinal macula (an area with a high cone content). Blood cells can get into the eye from capillaries that break down due to aging, high blood pressure, strokes. Under the action of osmotic pressure, these cells swell into balls.
19. Why do colored fabrics fade in the sun?
Ultraviolet radiation, being absorbed by organic molecules of paints, breaks molecular bonds. This results in loss of pigment.
20. If, looking at the TV screen, mumble "mmm" with your mouth closed, then dark lines will appear on the screen. "Mooing" in the appropriate tone can cause these bands to move up, down, or stand still. Why does “mooing” affect our vision so much”?
The image on the screen "flashes", as it is formed as a result of horizontal scanning by an electron beam. "Mooing" of the appropriate frequency causes vibrations of the head and eyes. In this case, the same repeating image periodically falls into the same area of the retina. This results in a stroboscopic image of the TV screen. If the frequency of the "moo" changes, then the image will move.
21. Covering one eye with sunglasses and looking with both eyes at a swinging pendulum, we will see that it describes an ellipse in space. Why is there an apparent three-dimensional image?
The apparent movement along the ellipse is explained by the fact that the perception of the pendulum by an eye closed by a dark filter lags behind by several milliseconds. The brain, comparing information from two eyes, "places" the pendulum either closer or further than the true position. Therefore, the oscillation appears to be two-dimensional.
22. Looking into the clear sky, you will see many moving points before your eyes. They are always there, but usually we do not pay attention to them. What are they and why do they move in jerks?
The brain "ignores" any motionless image in the eye, while the vessels in the retina and their shadows are motionless. Another thing is the shadows from blood cells moving through the capillaries; these shadows are visible as intermittently moving dots.
23. In low light, blue appears brighter than red, but in good light, red appears brighter than blue. Why does the relative brightness of colors depend on the level of illumination?
In strong light, vision is due to cones, and in low light, to rods. There are three types of cones that are sensitive to colors: red, yellow, blue. Rods are most sensitive to green light and least sensitive to red. If you increase the illumination, then vision switches from "rod" to "cone" (Purkinje color effect).
24. A fly settled on the front line of the camera lens. How will this affect the quality of the picture?
The fly will block some of the rays entering the lens, which will lead to a dimming of the picture.
25. Why does a person distinguish the outlines of objects worse in the evening than during the day?
In the evening, the pupils of a person dilate. But the lens is not a perfect lens. Images given by different parts of the lens are shifted relative to each other due to aberration. The more part of the lens "works", the more blurred the image.
26. Why does the Sun play different colors during sunrise and especially sunset?
The sun's rays at sunset and sunrise pass big way in the air. According to Rayleigh's theory, blue, blue and violet rays will scatter, and the rays of the red part of the spectrum will pass. Therefore, the Sun is painted in yellow, pink, red tones, the opposite side of the sky seems to be colored in blue with a purple tint. Sunrise gives a brighter and clearer picture, as the air becomes cleaner during the night.
27. If you look at the spotlight beam from the side, it seems to be curved. Is it so?
This perceptual error is due to the fact that the sky seems to us to be domed.
If all phenomena in this world had exclusively materialistic explanations, then the world, firstly, would be significantly impoverished from an artistic and aesthetic point of view, because many masterpieces would simply not be created, and, secondly, all the secrets of the universe would be revealed a long time ago.
However, this world is not as simple as it seems to materialists, and often its secrets are much deeper than one can imagine. And some of the secrets that emerge in our world have connections with other worlds, parallel or otherwise. This also applies to the shadow phenomenon.
HISTORICAL ASPECT
From the point of view of materialism, everything is very simple. A shadow is a sign of a blocked light. Or, speaking in a completely scientific language, a spatial optical phenomenon, which is expressed by a visually perceptible silhouette that appears on any surface due to the presence of an object between it and a light source. All in all. However, mainstream science is unable to explain why the shadow in many cultures causes awe and why there are so many rituals associated with shadows, and why they exist in almost all cultures throughout human civilization.
Probably because for many peoples shadows are dark entities that have their own nature. The shadow is often considered a double of a person and even a soul. In a number of languages, shadow and soul are denoted by the same word. Not without reason, in ancient Greek culture, hell was inhabited precisely by the shadows of the dead. In the Egyptian tombs of the New Kingdom, there are many images in which the black shadow of the deceased, accompanied by a bird-soul, leaves the grave. And in the "Book of the Dead" the following words are written: "Let the path be open for my shadow, for my soul, so that on the day of judgment in the other world they will see the great god." And the sanctuary of the sun god in Amarna was called “Shadow of Ra”.
There are also curious rock carvings from very archaic times, where people have strange elongated silhouette-like outlines. Scientists have suggested that in fact these are not people, but their shadows, while the very image of a person was forbidden.
Shadow-souls lead a semi-material existence and are able to interfere in the affairs of the living. It is from here that many funeral rites originate, designed, on the one hand, to help the dead, and on the other, to propitiate them. The absence of a shadow is a sign that the person is dead. That is why vampires do not have a shadow, and the devil himself is deprived of it, for he is an enemy of light in every sense. By the way, whoever makes a deal with him also loses his shadow. Witches, like vampires, have no shadow of their own. If, however, no ungodly deals were “formed”, but the person does not see his own shadow, he must die soon.
SUPERSTITATION OR...?
Until now, there are signs that have come to us from hoary antiquity. A lot of people try not to step into their own shadow, or they are careful not to fall into the shadow of another person. In some tribes, stepping on someone else's shadow is tantamount to a deadly insult. In ancient times, if a slave stepped on the shadow of his master, he was executed immediately, as they say - on the spot. And under the pharaohs, there was even a special person who made sure that the pharaoh did not step on his own shadow.
Among the Serbs, participants in ritual detours walked facing the sun so as not to accidentally trample their shadows. And in general, they distinguished between the shadow-soul (sen) and the shadow-double of objects (senka). Not only a person has a shadow-soul, but also trees, stones, animals, this is what gives them a special magical power.
The Bulgarians observed their shadows at the first rays of the sun on Ivan Kupala: if the shadow was intact, then the whole year would be healthy.
In Russia, they believed that the shadow could become a source of illness, because of which a person dries and withers, and in this case the shadow must be removed and destroyed. To do this, they put the patient against the wall, circled his shadow with chalk or chipped it with pins and measured it with a thread. Then the bottom was burned, and the pins were placed under the threshold, asking the shadow to take away the disease. Belarusians did the same thing: on a sunny day, they took the patient out into the yard, laid him on a board, circled him, and then burned the board.
There is another terrible rite that came from antiquity. We wonder why many ancient buildings are not destroyed. Yes, because then some living creature or its shadow was necessarily laid in the foundation. By the way, there was only one effect - the victim died, and the house became strong, and the disturbed spirit of the earth, receiving the victim, was appeased.
Sometimes builders specially lured an unsuspecting person to the construction site, secretly measured his shadow with a rope, and then walled up the measure with the first stone. The person whose soul-shadow was caught in this way died within 40 days, and his spirit settled in a new house as a guardian, next to the rope. And in order not to accidentally wall up their own shadow, the ancient masons never worked against the sun.
In Romania, stealing shadows is "practiced" to this day. And not so long ago, there was even a trial about the fact that one neighbor accused another of stealing his father's shadow. The plaintiff alleged that the defendant, when building his new house, "removed the shadow" from his father with a rope and laid it in the foundation of the dwelling, as a result of which the man, strong and not ill, died unexpectedly. Eyewitnesses in court also claimed that the ghost of the deceased now roams around the house where his captured soul is buried.
The church shadow was considered very good, which is why burials under the shadow of temples were the most honorable, because the deceased was under the highest patronage.
SHADOW TRAPS
Nowadays, many magical rites have leaked to the masses, where the main role is played by the shadow. So, if you do not want to part with your loved one, pin his shadow with a pin to the curtain or to your clothes. You can also scrape the dust in the place where the shadow of the faithful fell, collect it in a bottle and wear it near the heart, or you can simply circle the shadow of a loved one in order to avoid separation.
If you want to win an argument, step on the throat of your opponent's shadow. If you want to get rid of an evil enemy in the office, catch his shadow: glue or seal it with wax, then sweep the floor, "noticing" the shadow on the scoop, and then throw out the garbage, after spitting on it.
Just contacting the world of shadows, remember that they are insidious creatures, and you can expect anything from them. They do not forgive games with themselves. And if the shadow begins to affect you, say the ancient spell: "Shadow, know your place!"
SCIENCE AND SHADOWS
As for science, a curious experiment was recently conducted by British and Italian scientists: they acted with a wide variety of stimuli on ... the shadows of the subjects' hands. And a curious picture emerged: the participants in the experiments reacted to the stimuli of the shadows of the hands in exactly the same way as if these stimuli acted on the hand itself.
"The results confirm the intuitive connection that people feel in relation to their shadow contour," Professor Margaret Livingston summed up the experiment. - All of us in childhood experienced a reluctance to step on our shadow. This means that the brain, in determining the position of the body in space, uses the visual cues that it receives not only from the limbs, but also from the shadow.
Or, perhaps, the brain stores information that our ancestors knew about the mystical properties of the shadow and how to behave with it correctly? In psychology, the term "shadow" refers to the intuitive part of the soul, which is often suppressed.
Psychologists say that the shadow is a projection of the reverse side of the personality, and if you are good, your shadow is terrible, and vice versa. In dreams, the shadow hides under the guise of monsters or defective characters. Especially often they appear during the formation of personality or serve as a sign that it is necessary to change one's own behavior.
MYSTICITY AND PHYSICS
And then there are the so-called shadows of Hiroshima. This, on the one hand, is precisely explained from the point of view of physics. The shadows of Hiroshima are an effect that occurs as a result of the action of light radiation during a nuclear explosion, and represents silhouettes on a burnt out background in places where the body of a person or animal interfered with the spread of radiation.
Shadows of Hiroshima
The shadow of a man who was sitting on the steps of the stairs in front of the bank entrance at the time of the explosion, 250 meters from the epicenter.
Shadows on the bridge
The shadow of a standing man on the steps
In Hiroshima, the epicenter of the explosion fell on the Aion Bridge, where the shadows of nine people remained. But it may turn out that the intense radiation not only imprinted the silhouettes of people to the surface, but also caught their shadows, and even their souls, like the same pin or adhesive tape, chaining them forever to the damned city.
Another case, also related to the war, defies rational explanation. In Germany, there is a small town of Bietigheim, where terrible things happen: once every ten years, human shadows appear on the walls of houses, moving as if they were alive.
This happened in 2001, in 1991, and possibly earlier. These shadows appear on the anniversary of a terrible event - the mass execution of Jews in 1941, when thousands of people were taken out of the city and destroyed. What is this - an otherworldly reminder to the living, a city curse or a place of shadows?
Aventina ROSSI
Even in ancient times, scientists were interested in the nature of light. What is light? Why are some objects colored and others white or black?
Empirically, it was found that light heats the bodies on which it falls. Therefore, it transfers energy to these bodies. You already know that one of the types of heat transfer is radiation. Light is radiation, but only that part of it that is perceived by the eye. In this regard, light is called visible radiation.
Since light is radiation, all the features of this type of heat transfer are inherent in it. This means that energy transfer can be carried out in a vacuum, and the radiation energy is partially absorbed by the bodies on which it falls. As a result, the bodies heat up.
The bodies from which light emanates are sources of light. Light sources are divided into natural and artificial.
Natural sources of light are the Sun, stars, atmospheric discharges, as well as luminous objects of the animal and plant world. It can be fireflies, rotten, etc.
a - firefly; b - jellyfish
Artificial light sources, depending on what process underlies the production of radiation, are divided into thermal and luminescent.
Heat sources include light bulbs, gas burner flames, candles, etc.
a - a candle; b - fluorescent lamp
Luminescent sources are fluorescent and gas light lamps.
We see not only sources of light, but also bodies that are not sources of light - a book, a pen, houses, trees, etc. We see these objects only when they are illuminated. The radiation coming from the light source, hitting the object, changes its direction and enters the eye.
In practice, all light sources have dimensions. When studying light phenomena, we will use the concept point light source.
If the dimensions of the luminous body are much smaller than the distance at which we evaluate its action, then the luminous body can be considered a point source.
Huge stars, many times larger than the Sun, are perceived by us as point sources of light, since they are located at an enormous distance from the Earth.
Another concept that we will use in this section is light beam.
A light beam is a line along which energy travels from a light source..
If an opaque object is placed between the eye and some light source, then we will not see the light source. This is explained by the fact that in a homogeneous medium, light propagates in a straight line.
The rectilinear propagation of light is a fact established in ancient times. The founder of geometry Euclid (300 BC) wrote about this.
The ancient Egyptians used the law of rectilinear propagation of light to set up columns in a straight line. The columns were arranged in such a way that all the others were not visible because of the column closest to the eye (Fig. 122).
Rice. 122. Application of the law of rectilinear propagation of light
The straightness of the propagation of light in a homogeneous medium explains the formation of shadows and penumbra. The shadows of people, trees, buildings and other objects are well observed on Earth on a sunny day.
Figure 123 shows the shadow obtained on the screen when a point light source S illuminates an opaque ball A. Since the ball is opaque, it does not transmit light falling on it. The result is a shadow on the screen.
Rice. 123. Getting a shadow
A shadow is that region of space that is not hit by light from a source..
Such a shadow can be obtained in a dark room by illuminating the ball with a flashlight. If we draw a straight line through the points S and A (see Fig. 123), then point B will also lie on it. The straight line SB is a ray of light that touches the ball at point A. If the light did not propagate in a straight line, then the shadow could not be formed. We got such a clear shadow because the distance between the light source and the screen is much larger than the size of the light bulb.
Now let's take a large lamp, the dimensions of which will be comparable with the distance to the screen (Fig. 124). A partially illuminated space is formed around the shadow on the screen - penumbra.
Rice. 124. Getting penumbra
Penumbra - this is the area into which light enters from part of the light source.
The experiment described above also confirms the rectilinear propagation of light. Since in this case the light source consists of many points and each of them emits rays, there are areas on the screen in which light from some points enters, but not from others. This is where the penumbra is formed. These are areas A and B.
Part of the screen surface will be completely unlit. This is the center area of the screen. Here there is full shadow.
The formation of a shadow when light falls on an opaque object explains such phenomena as eclipses of the Sun and Moon.
When moving around the Earth, the Moon can be between the Earth and the Sun or the Earth - between the Moon and the Sun. In these cases, solar or lunar eclipses are observed.
During a lunar eclipse, the Moon falls into the shadow cast by the Earth (Fig. 125).
Rice. 125. Lunar eclipse
During a solar eclipse (Fig. 126), the shadow of the Moon falls on the Earth.
Rice. 126. Solar eclipse
In those places of the Earth where the shadow fell, a total eclipse of the Sun will be observed. In places of penumbra, only part of the Sun will be covered by the Moon, i.e., there will be partial solar eclipse. In other places on Earth, the eclipse will not be observed.
Since the movements of the Earth and the Moon are well studied, eclipses are predicted many years in advance. Scientists use each eclipse for a variety of scientific observations and measurements. A total solar eclipse makes it possible to observe the outer part of the Sun's atmosphere (solar corona, Fig. 127). Under normal conditions, the solar corona is not visible due to the dazzling brilliance of the Sun's surface.
Rice. 127. Solar corona
Questions
- What is a beam of light?
- What is the law of rectilinear propagation of light?
- Which phenomenon is proof of the rectilinear propagation of light?
- Using figure 123, explain how a shadow is formed.
- Under what conditions is not only a shadow, but also a penumbra observed?
- Using Figure 124, explain why some areas of the screen are in partial shade.
Exercise 44
The task
- In a piece of thick cardboard, make a hole with a diameter of 3-5 mm. Place this piece of cardboard about 10-15 cm away from the wall opposite the window. On the wall you will see a reduced, inverted, dimly lit image of a window. Obtaining such an image of an object through a small hole is another proof of the rectilinear propagation of light. Explain the observed phenomenon.
- To get an image of an object with a small hole, make a device called a "camera obscura" (dark room). To do this, glue a cardboard or wooden box with black paper, make a small hole in the middle of one of the walls (about 3-5 mm in diameter), and replace the opposite wall with frosted glass or thick paper. Get an image of a well-lit object with the help of a made camera obscura. Such cameras used to be used for photographing, but only stationary objects, since the shutter speed had to be several hours.
- Prepare a presentation on Solar and Lunar Eclipses.
1276. Give examples of light sources known to you.
Sun, stars, fluorescent lamps, candle, el. a lamp, an electro-ray tube of old TVs, the glow of insects and fish, luminous paints.
1277. Give examples when not only a shadow is observed, but also a penumbra
Penumbra from a celestial body can be observed, for example, during a partial eclipse of the Sun, when the observation point falls into the penumbra formed by the Moon in a stream of sunlight.
An object in a room with multiple light sources.
1278. Due to what phenomenon is a shadow formed?
The shadow is formed because the rays move in a straight line, without bending over objects.
1279. Give examples proving the rectilinear propagation of light.
1280. If the lamp hangs directly over the table, will the same shadows be obtained from a pencil lying on the table, and from standing upright?
Not the same. From a vertical standing pencil, the shadow will be in the form of a dot. From a horizontally lying shadow in the form of a line.
1281. During the operation, the shadow from the hands of the surgeon should not cover the site of the operation. How should the lamps be positioned for this?
Light sources must be placed around the perimeter of the room from different sides. Any shadow from the hands should be illuminated by another lamp.
1282. In sunlight, a vertical stick 1.5 m high casts a shadow 2 m long, and a factory chimney casts a shadow 50 m. Determine the height of the factory chimney.
1283. In sunlight, the shadow of an object is equal to the height of the object. At what angle to the horizon is the Sun?
Angled 45°
1284. Have you ever observed on a sunny day on a path under a tree covered with dense foliage, rounded bright spots? Why are they formed and what are they?
These are areas of penumbra and light. Formed by the passage of light through the foliage and the reflection of light from the leaves and branches.
1285. The ratio of the diameters of the Moon and the Sun is approximately 1:400. During the new moon, the distance between the centers of the Moon and the Sun is approximately 150,000,000 km. What is the length of the shadow cone cast by the moon at new moon?
1286. The radius of the Sun is equal to 110 terrestrial radii. The radius of the Earth is 6370 km. The distance from the center of the Earth to the center of the Sun is about 23,900 Earth radii. What is the length of the cone of shadow cast by the globe when illuminated by the Sun?
1287. Rays from a lantern at a distance of 40 m pass through a small hole in the screen. On the opposite wall, at a distance of 7.5 m from the screen, an image of a lantern is obtained. Image size 0.75 m. Determine the size of the lantern.
