Light
Class 7 | Science — Physics | Unit 1
Class 7 Unit 1 Science
Learning Objectives
The students will be able to understand that light is an energy. differentiate natural and artificial light sources. understand rectilinear propagation of light. understand formation of shadows know reflection of light and its types know the laws of reflection. understand the properties of the images formed in a plane mirror. understand dispersion and spectrum. understand synthesis of colours
Introduction
When you enter a dark room, nothing is visible. The moment you switch on the light, everything in the room becomes visible. How do we see things with our eyes? When you look at this book, the light falling on the book is reflected and enters your eyes. Light is a type of energy that helps us to see all the things around us. Light can be detected by the human eye. We all know that light is essential for vision. Let us see more about light in this chapter.
Do You Know?
Light is the only source of energy for plants. So, they entirely depend on light. People and animals derive energy from carbohydrates, protein, and fat through their food. Plants produce food using the energy from Sunlight, carbon-di-oxide and water by the process called as Photosynthesis. Sunlight acts a vital role in the process of photosynthesis.
Sources of Light
Objects which are able to emit light are known as light sources. Light rays can come from different sources. There are two types of sources of light.
TYPE ONE: Natural sources of light.
TYPE TWO: Artificial sources of light.
Natural Sources of light
Sources which emit light naturally are known as natural sources of light. The Sun is the primary and the major source of natural light. Stars also produce light, in the same way as the Sun do. However, as they are much farther away than the Sun, the light from them are too weak. The moon provides light, particularly in the night. Some living organisms have the ability to produce light named by bioluminescence. It is the effect of certain chemical reactions occurring in the organism. Fireflies, jellyfish, glow worm, certain deep sea plants and some microorganisms can emit light naturally.
Artificial Sources of light
Apart from the natural sources, light can also be produced artificially. The different light sources that are able to produce light artificially can be put under the following categories.
Do You Know?
Is the moon a luminous object? The moon provides light as well, but it cannot produce light by its own. The light emitted by the Moon is the light of the Sun reflected towards the Earth. When we see the Moon, we see only the Moon's lighted part. Thus, half of the moon is always facing the Sun and receiving light from it. Hence, we receive light from the moon.
Artificial sources are manmade light sources such as flame of candle, incandescent lamp, neon lamp, Sodium lamp etcetera.
1. Incandescent Sources
When certain objects are heated to a high temperature, they begin to emit light. The glowing of hot iron rod is a kind of Incandescent light.
Example: Candle, incandescent lamp.
2. Gas Discharge Sources
Passing electricity through certain gases at a very low pressure (discharging) can produce light.
Example: Neon lamp, Sodium lamp
Do You Know?
We often use a kind of gas-discharge lamp that uses fluorescence to produce visible light. The electric current in the gas excites mercury vapour, which produces short wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow in visible light.
Properties of light
In this section, we shall examine some properties of light. Light has some fundamental properties as mentioned below.
- Rectilinear propagation of light
- Reflection
- Speed
- Interaction of light with matter
- Types of material according to Permeability
- Formation of shadows
- Plane mirror and images
- Spectrum
The path of light
How does light travel? Have you ever seen the scene of light penetrating through the branches of trees in denser forest? Have you ever seen the path of sunlight entering through the hole of a cement grill building? Have you ever seen the path of a laser light?
Activity 1
Requirement: Three empty match boxes, pin, candle, and wooden blocks.
Procedure: Arrange empty match boxes and wooden blocks as shown in the figure. First, you make a hole in the inner tray of each match box such that all three holes are in the same spot. Arrange the match boxes as shown in figure. Now, adjust the three inner trays in such a way that the three holes are in a straight line. Place a lighted candle at one end of this arrangement and try to see the flame of candle from a hole at the other end. Is the flame visible? Now, arrange the trays such a way that they are not at the same height. Try to see the flame. Is it visible? What does this activity tell you about the path of light?
Light travels in straight line, it cannot bend the path itself. This is called as the rectilinear propagation of light. This is one of the most important property of light.
Do You Know?
Al-hasan-Haytham was a scientific thinker who made important contribution to the understanding of vision, optics, and light. He observed that light coming through a tiny hole travelled in straight lines and projected an image onto the opposite wall. Based on such experimentation, he concluded that vision is accomplished by rays coming from external luminous sources and entering the eye, rather than through rays emitted from the eye as was then commonly believed. He is the first one to experiment with light and found important properties like the rectilinear propagation of light.
Pinhole Camera
Pinhole camera is a simple device which helps us to understand about the rectilinear propagation of light. The above picture shows a model of a pin hole camera. O is small hole by a pin. XY is the object and Y prime X prime is the image of XY. As light travels in straight line, one light ray from X travels along the X O strikes the screen X prime.
Activity 2 - Make your pinhole camera
Requirement: Two rectangular pieces of thick paper, carbon paper, a semi-transparent paper, adhesive
Procedure: Make two tubes using thick paper as shown in figure. One tube should be slightly smaller in diameter so that it can slide into the other tube without leaving much gap between the tubes. Fix a carbon paper to one side of the tube of greater diameter. Make a hole with a pin at the center of the carbon. Close one end of the second tube with the butter paper. Slide the smaller tube into the bigger one such a way that the butter paper is inside. Keep a lighted candle on a table and look through the hole with black side towards the candle. If you go closer to the candle, you will see a smaller, but brighter image. You can also change the image size by adjusting the tubes. Use the pin-hole camera to see things in sun light outside the window and see how good an image you get. What are your observations about the image? Is it straight, inverted, bright and sharp?
In similar way, another light ray staring from Y and travels along Y O strikes the screen Y prime. Similarly, all the rays in between X and Y fall on the screen between Y prime and X prime. Thus, Y prime X prime becomes the image of X Y. The image produced is temporary if a simple paper is used. The image can be made permanent if the paper is replaced by a photographic plate.
Reflection
A mirror reflects our face. A still water body like a pond reflects the scenery around it. When we see our face in the mirror, we see the light rays from our face bouncing off the surface of the mirror. How the rays of the light are reflected? Take a plane mirror. Cover it with black paper. Cut a small slit as shown in the figure. If You shine light on the mirror from a torch light or sunlight, you will get a small ray of light. We can use this to study the properties of light.
Do You Know?
Before the advancement of camera, Pinhole camera was used to photograph movement of the sun over a long period of time. This type of photography is known as solography and be used for observing and recording solar eclipses. And it was also used to take photograph of stationary objects.
Place a blank white sheet on a level groundout in the open. Choose a place where partly the sheet gets sunlight and partly it is in shadow. Hold the mirror with the slit facing the sun. You can see a straight ray of light reflected from the slit on the paper. Hold another mirror to reflect this ray. Observe well. The light falling on the mirror is called as incident ray and the light reflected is called reflected ray.
Relationship between the incident ray and reflected ray? Draw a straight-line ABC and angles as shown in the figure above. The line 1 is at 60 Degree from BD, 2 at 30 Degree from BD. Now, the line 4 at 60 Degree from BD and line 3 at from 30 Degree BD. The line BD is perpendicular to ABC. Hold the mirror along the line ABC. Use the mirror with slit and make a ray go along line 1 and reach the mirror at point B. Observe where the reflected ray is. Is the reflected ray go along 4? Now, try keeping the mirror with slit and make the incident ray go along line 2. Now do we see that the reflected ray is a long line 3? Line BD, which is perpendicular the mirror surface is called as normal. The angle between the incident ray and the line BD is called angle of incident. Similarly, the angle between reflected ray and the normal is called as angle of reflection. Can you make out relationship between the angle of incident ray and the angle of reflected ray? Yes. Is it not obvious that the angle of incident is same as the angle of reflection?
Terms used in reflection of light
Incident ray: The ray of light that falls on the surface of the reflection materials. In the figure, PO is the incident ray.
Reflected ray: The ray of light that comes from the point when the incident ray falls on the reflection material. In the figure, O Q is the reflected ray.
Point of incidence: The point of which are incident rays strike the reflecting surface is the point of incidence. In the figure 'O' is the point of incidence.
Normal: The perpendicular line drawn from the point of incidence to the plane reflecting the surface is called normal. In figure, O N is the normal.
Angle of incidence: The angle formed between the incident ray P O and the normal 'O N' is angle of incidence. It is denoted by angle i
Angle of reflection: The angle formed between the reflected ray O Q and the normal O N is angle of reflection. It is denoted by angle r
Laws of reflection
Point number one: The angle of incidence is always equal to the angle of reflection angle i equal to angle r
Point number two: The incident ray, the reflected ray and the normal at the point of incidence lie on the same plane.
Example 1
In the figure, the incident ray makes 27 degree with the normal, then find the angle of reflection.
Solution: Angle of incidence equal to 27 degree
According to the laws of reflection, the angle of reflection equal to Angle of incidence equal to 27 degree
Example 2: A light ray strikes a reflective plane surface at an angle of 43 degree with the plane surface.
Question number one: Find the angle of incidence.
Question number two: Find the angle of reflection.
Question number three: Find the angle between the incident and the reflected ray.
Question number four: Find the angle between the reflected ray and the plane surface.
Solution: We use the diagram shown below to answer the questions.
a) Angle of incidence angle i equal to 90 minus 43 equal to 47 degree
b) angle of reflection angle r equal to angle i equal to 47 degree
c) angle i plus angle r equal to 47 plus 47 equal to 94 degree
d) angle x equal 90 minus angle r equal 90 minus 47 equal to 43 degree
Types of reflection
On a mirror we can see our image, but not on the wall. Why? Both the surface reflects light. Only because the reflected light comes to our eyes, we are able to see it. If the wall was not reflecting light, then we cannot see it. We saw earlier that the light reflects off surfaces in a very predictable manner, in accordance with the law of reflection. The laws of reflection holds good for all surfaces irrespective of the shape. Vertical surfaces, angled surfaces, and even over the curved surfaces, the laws of reflection hold good. As long as we can draw the normal, perpendicular to the surface at the point can be drawn, the angle of incidence at that point will be equal to angle of reflection. The law of reflection is always observed regardless of the orientation of the surface. If the surface is smooth, and flat, all points on it have the normal in the same direction. Therefore, a set of parallel rays striking the surface will be reflected at an angle, but the rays themselves will still remain parallel to each other. However, consider a surface which is not smooth. Such as the surface of a wall. What happens when the light ray hits the rough surface? Roughness of the wall means that each individual ray meets a surface which has a different orientation. The diagram below depicts the case. Five incident rays labelled as A, B, C, D and E approach a surface. The normal line at each point of incidence is shown in black and labelled with an N. In each case, the law of reflection is followed, resulting in five reflected rays labelled A prime , B prime , C prime , D prime and E prime . While the incident rays were parallel to each other, the reflected rays are going in different directions. The result is that the rays of light are incident upon the surface in a concentrated bundle and are diffused upon reflection. Broadly, we can say that there are two types of reflection. If the surface is smooth then we have specular reflection. The parallel light rays striking the surface gets reflected, yet individual reflected rays remain parallel. If the surface is rough, then we have diffused reflection. Light rays, after reflection, go in many directions. In fact, during the day, our classroom is illuminated by sunlight. Walls and floors are exposed to diffuse reflection. Suppose walls were smooth mirror like. Then sunlight entering through the window will get bounced by the floor at an angle above towards the roof. And it will never get reflected to left or right. That is left and right walls will remain dark. However, walls and floors are not smooth surfaces. Therefore, incident light from the window get bounced in all directions that the whole room is illuminated with diffused light.
Types of beam of light
Generally light is not a single ray, but a bundle of rays which are called as a beam of Light. A light beam can be a bundle of parallel rays, convergent rays, or divergent rays. Let us look at the light coming from the Sun. The rays of sunlight are parallel. Often the headlight of car gives parallel rays. However, look at the rays of light coming out of a candle. Light rays go in all directions, from the candle fire. These rays are divergent. Light rays from a flashlight is also divergent. Using lenses, we can converge light rays. Using a lens, you can focus sunlight at a point. That is what we are making the light rays to converge.
Speed of light
When lighting a bulb in a dark room, light spreads the whole room quickly. This is because the light travels very fast. Light travels three lakh kilometers per second in air or vacuum. In theory, nothing can travel faster than light
Interaction of light with matter
Take a piece of clear glass, a paper and a metal sheet. Shine a light from one side of each object and see if the light penetrates on the other side. Readily, we can see light enters and comes out of the other end of clear glass, whereas the light is bit dim through a paper. Light does not pass through metal sheet. Depending upon permeability, materials can be classified into three categories.
Transparent Material
Materials that allow light to pass through completely are known as transparent material.
Example: Eye glasses, clear drinking glass, clear water, face glasses used in buses.
Translucent Material
Objects that allow light to pass through partially are called translucent material. For Example, we cannot see the image of someone who stands behind a rough window glass, because it allows only a part of light from the person.
Opaque Material
Materials that are not able to allow light to pass through, are called opaque material.
Example: Wall, thick card board, stone, etcetera.
Activity 4
Let's categorize transparent material, translucent material and opaque material among the given materials (Clear plastic ruler, cellophane paper, some water in a glass jar, tissue paper, drinking glass, beaker, tap water, kerosene, coconut oil, note paper, card board, milk, diluted milk, aluminum foil, thick coloured plastic lid, rough glass piece, measuring glass with water, wooden piece)
Place all the materials given above in the dark room. Focus a torch light on one side of each materials. Inspect the light coming out at the other side of each material and then classify the materials in the table.
| Serial number | Transparent material | Translucent material | Opaque material |
|---|
| 1 | | | |
| 2 | | | |
| 3 | | | |
| 4 | | | |
| 5 | | | |
Shadows
How are shadows formed? As we saw earlier, light is obstructed by certain materials. Light travels in a straight line. Hence it cannot go around such objects. That is why we see shadow. Shadow is always against, opposite side of light source. It is caused by opaque objects that stop light from propagating.
Parts of shadow
When an opaque object is placed in the path of light from a point source, a uniform dark shadow will appear on the screen. This is shadow is called as umbra. When an opaque object is placed in the path of light coming from a broad source of light, a small umbra will appear on the screen and an illuminated shadow area appears around umbra. This illuminated shadow area is called as penumbra. The penumbra always surrounds the umbra. The umbra is the darkest part of a shadow. In this part, light rays are completely prevented by the opaque object. The lighter shade of shadow is the penumbra.
Properties of shadow
Property one: All objects do not form shadows. Only opaque objects form shadows
Property two: Shadows will be formed in the opposite side of light source
Property three: It cannot be determined the characteristics of an object by its shadow.
Property four: the shadow will be always darker, whatever may be the color of light rays
Property five: Light source, opaque object is shadow all are in a straight line.
Property six: the size of shadow depends upon the distance between light source and object and the distance between object and the screen.
Activity 5
| Arrangement | Activity | Observation | You Learn |
|---|
| Place a lighted bulb in front of a rectangular cardboard with a hole at the center | | A shadow with a spotlight appears in the screen. Light rays are passing only through the hole and are not allowed by the remaining part of cardboard | |
| Place a pencil in the path of light ray coming from a bulb | | A shadow of pencil appears in the screen | The size of the shadow is proportional to the size of the opaque objects |
Activity 6
Requirement: A white screen, a cylindrical opaque object and three bulbs in different sizes.
Use the three different size lamps and examine the umbra and penumbra formed. Keep the distance between the lamp and the cylinder, cylinder and the screen same. As the size of the lamps grow smaller, the umbra region begins to enlarge. If the size of the lamp is a point, then there will be no penumbra. There would be only umbra shadow. Can you tell what the reason is for that?
Eclipses
An eclipse is an incident, when any astronomical object is partially or fully obscured due to the placement of another astronomical object in the presence of light. Thus, solar and lunar eclipses are occurring that are due to the property of light known as the rectilinear propagation of light.
Solar eclipse: Solar eclipse occurs, when the moon arrives between the sun within bracket S and the earth within bracket E. The shadow of the moon appears on the earth at A as shown in picture. Hence, those who are at the region A are unable to see the Sun instantly. This is solar eclipse. But those who are at the region B and C are able to see the sun partially.
Lunar eclipse: Lunar eclipse occurs, when the earth within bracket E comes between the sun within bracket S and the moon within bracket M. The earth prevents light coming from the sun and makes shadow on the moon. This is lunar eclipse.
Activity 7 - Try This
Plane Mirror and Reflection
A polished or smooth surface with in bracket like glass which forms image by reflection is known as mirror. A plane mirror is a mirror with a flat reflective surface. A plane mirror makes an Image of objects in front of it.
Do You Know?
Optical fibre is a device that works on the principle of total internal reflection by which light signals within bracket huge data can be transmitted from one place to another place with a negligible loss of energy in a very short time. It consists of a cable having one or more thin flexible fibres with a glass core through which light signals can be sent. Optical fibre can be twisted and bent easily. When a light a ray of light is incident at one end of the core of optical fibre, it suffers total internal reflection at the many places inside the fibre and emerges at the other end with negligible loss of energy. The data or information in the form of pulses of light, can be sent through bundles of optical fibres. Optical fibres have become very important in high-speed communications, such as cable TV and high speed broadband services. Fiber optic cables are able to carry more signals than traditional copper cable telephone lines.
Real and virtual images
We have seen images being formed in a pinhole camera and a mirror. Can we see what is different in both of these images? Firstly, the image of the pinhole camera was formed on a screen. While the image made by the mirror is not obtained on a screen. The images that are obtained on a screen are called 'real image' and that which cannot be obtained on a screen 'virtual image'. Also notice that the image on pinhole camera was upside down. While the mirror image was upright.
Properties of Image formed in a plane Mirror
- Image formed in a plane mirror is upright
- Image formed in a plane mirror is virtual
- The image is of the same size as the object
- The distance of the image from the plane mirror is equal to the distance of the object from the mirror
- Image is laterally inverted
Activity 8
There are eight letters in the word. EINSTEIN
1. Write the word in front of a plane mirror shown in diagram.
2. Write down how these letters appear in the mirror.
3. How many of these letters appear to be different, when the word is reflected?
4. Write down the letters that appear to be the same.
Color
Color of sunlight
Light is a form of energy in the form of a wave that simulates that retina of our eyes. Visible light is a spectrum of a number of waves with different wavelength range from 400 nanometre to 700 nanometre within bracket 1nanometer equal to 10 power within bracket minus 9 meter each wave has a definite wavelength represents a particular color. The band of visible light is VIBGYOR.
- V Violet
- I Indigo
- B Blue
- G Green
- Y Yellow
- O Orange
- R Red
Violet color has shorter wavelength and red color has longer wavelength. When light ray of wavelength within bracket Color strikes the retina of our eye, our brain perceives that specific color. When all colors of visible light strike the retina of our eye at the same time, our brain perceives white. This shows, white is not a color at all. But it is the combination of all the colors of the visible light spectrum. If all the wavelengths with in colors of visible light spectrum give appearance of white similarly, the observe of all their wavelengths of visible light, will lead appearance of black.
Do You Know?
Why is the word "AMBULANCE" written backwards in ambulance vehicle? This is due to lateral inversion phenomenon due to which the left side of an object appears to be right side of the object in its image in a reflecting medium within bracket mirror. so that drivers see the word the right way around in their rear view mirror
What is prism
A prism is an object made up of a transparent material, like glass or plastic that has at least two flat surfaces that from an acute angle. With in bracket less than 90 degree
Activity 9
We have seen that white light is made of different colours and we can split white light. Is it possible to do the reverse? That is, can you get white colour by mixing colours? Try this activity. You need oil pastel and white paper. Take different oil pastel colours. Choose colours which are exactly seen on the rainbow. Apply colours over each other on a white paper. Did you get white colour?
Difference between the images formed in Pinhole camera and Plane mirror
| Images formed by Pin hole camera | Images formed in Plane mirror |
|---|
| The image is real | The image is virtual |
| The image may not be equal to the size of the object | The image is equal to the size of the object |
| The image is inverted | The image is erect |
Activity 10 - Let's make a rainbow
You must surely have seen a rainbow in the sky. Why don't you try making one at your classroom? Place a flat but deep pan. Place this pan where there is direct sunlight. Place a plain mirror in the pan as shown in the diagram, so that you see sunlight reflected on you ceiling or on a white wall. Next slowly pour water into the pan. At particular level of water, you will get a beautiful rainbow colors on the wall. If the colours are not clear adjust the position of the mirror to bring it into focus. This arrangement of colours in sunlight is called spectrum.
Do You Know?
Why danger lights in vehicles are red in colour?
1. Red colour is scattered the least by air molecules.
2. Red colour has the highest wavelength of all the other colours. So red colour is able to travel the longest distance through air, fog.
When white light is passed through a prism as shown in the figure, the colors of the rainbow emerge from the prism. This phenomenon is called dispersion of light, and the band of colour is called as spectrum.
Newton Disc
Newton suggested a process of mixing different colors to make white color by setting an arrangement as shown figure below. Newton Disc is a card board disc with seven equal sectors colored red, yellow, orange, green, blue, indigo and violet. When the disc turned quickly, the retina receives the sensation of the spectrum simultaneously and disc appears white. Using this disc, one can explain that white is a combination of VIBGYOR.
Activity 11
You need Gelatin papers of Red, Blue and Yellow. Fold each gelatin paper three times and look different colour objects listed below through each folded paper. Observe what colour each object has. Write your observations in the table.
| object | Original colour of object | Colour through Red gelatin paper | Colour through Blue gelatin paper | Colour through Yellow gelatin paper |
|---|
| Blue sky | | | | |
| Orange flower | | | | |
| Yellow banana | | | | |
| Leaves | | | | |
| Brown trouser | | | | |
| White shirt | | | | |
| Black board | | | | |
We know that white shirt will reflect white light and we have seen that white light consists of different colours. When we look at the white shirt through the yellow gelatin paper, we see it as yellow in colour. From this, we can say that the yellow gelatin paper did not allow any other colour except yellow to pass through. Similarly, we conclude that red gelatin paper allows only red light and blue gelatin paper allows only the blue light.
Synthesis of colors
Synthesis of color is the method of creating color by mixing various proportions of two or three distinct colors of light. These distinct colors are Red, Green and Blue called as primary colors. Equal proportions of two primary colors create a secondary color. Magenta, Cyan and yellow are called secondary color.
1 Red Plus 1 Blue Equal to Magenta
1 Blue plus 1 Green Equal to Cyan
1 Green plus 1 Red Equal to Yellow
Equal proportions of all three primary color create white.
1 Red plus 1 Blue plus 1 Green Equal to white
EVALUATION
Exercise A. Choose the correct option
1. Light travels only in a ______. It is because of this property that ______ are formed
a. curved line, shadows b. straight line, shadows c. straight line, reflection d. curved line and then straight line, shadows
2. Light that hits a mirror gets ______
a. Transmitted b. Reflected c. Absorbed d. Refracted
3. ______ Surface reflects the light well.
a. water b. compact disc c. mirror d. stone
4. Light is a form of ______
a. matter b. energy c. medium d. particle
5. You can see your image in polished floors, but not in wooden table because ______
a. regular reflection takes place in wooden table and irregular reflection in polished floor. b. regular reflection takes place in polished floor and irregular reflection in wooden table c. regular reflection takes place in both polished floor and wooden table d. irregular reflection takes place in both polished floor and wooden table
6. Choose the translucent substance from the following
a. glass b. wood c. water d. Clouds
7. Reflection occurs, when the light ______
a. about to reach a surface b. approaches a surface c. passes through a surface d. None of these
8. The image formed by a pinhole camera is inverted because
a. light travels in straight lines b. light rays become laterally inverted as they pass through a pinhole camera c. light rays pass through the pinhole d. light rays get reflected
9. Which of the following facts explain how shadows are formed?
a. Light travels in straight lines b. Opaque bodies do not allow light to pass through them c. Reflection occurs at a smooth surface like mirrors d. Lateral inversion happens a. both A and B b. both A and D c. both B and C d. only A Exercise B. Fill in the blanks
1. A plane mirror produces a image
2. A reflection helps us to see the objects
3. The light ray gets when it falls on any polished surface.
4. Sunlight is a blend of colors
5. The splitting of white light into seven colors is called
6. The moon sun light
7. The sunlight can be split into its constituent colors using
8. Reflection of light from rough surface is called reflection
Exercise C. Say TRUE or FALSE 1. The image of right hand in a plane mirror looks like a left hand 2. Rainbow is formed by dispersion of which light by water drops 3. The image formed by the plane mirror is laterally inverted, hence the image seen through the periscope is also laterally inverted 4. We see planets because they reflect light from the sun 5. We see a book because it reflects the light that falls on its surface 6. The image formed in a pinhole camera is always inverted 7. The image formed in a pinhole camera is always the same size as the object 8. The image formed in a plane mirror is upside down 9. A plane mirror is opaque 10. A shadow is formed on the same side of the object as the source of light. 11. we are able to see things around us with the help of regular reflection 12. After passing through a prism, white light splits into a band of seven colors Exercise D. Match the following
| 1. Rectilinear propagation | |
| 2. Plane Mirror | |
| 3. Firefly | |
| 4. The Moon | |
| 5. Wide light source | |
| 6. Regular reflection | |
| 7. The sun | |
| 8. Band of seven colors | |
Options: Primary source of light, Non luminous object, Periscope, Pinhole camera, Penumbra, luminous object, Glossy surface, Spectrum of light
Exercise E. Answer the following questions in short
1. With the help of a diagram, state the laws of reflection
2. Figure shows a pencil placed above a mirror
a. Draw its image formed by the mirror
b. Show how light rays from the object are reflected at the mirror to form the image for the eye.
3. A person is looking at the image of a tree in a mirror placed 3.5 m in front of him. Given that the tree is at 0.5 m behind his eyes. Find the distance between the image of the tree and his eyes. What are needed to see an object?
4. What are luminous objects?
5. Is the moon a luminous object?
6. What are the three types of materials based on the absorption of light?
7. What are the parts of shadow?
8. What are the properties of shadow?
9. What is plane mirror?
10. What is prism?
11. What do you mean by visible light?
12. Write the items given here in the correct column (Stars, brick walls, plants, mirror, planets, electric light bulb, candle)
| Luminous objects | Non-luminous objects |
|---|
| |
| |
| |
13. A boy of height 1m 45 cm is standing in front of a long mirror at a distance of 2 m. From this information, fill up the following sentences:
a. The distance between the boy and his image is
b. The height of the image is
c. When the boy moves 1m forward, the distance between him and his image is
14. Draw a diagram of a pin hole camera showing the rays of light passing between the Object and its image
15. Why is the writing on the front of an ambulance back to front as shown in the picture?
16. Explain with examples, why some capital letters look the same in a mirror but others are reversed.
17. Two plane mirrors M1 and M2 are placed perpendicular with each other, as shown in figure. The ray AB makes an angle 39° with the plane mirror M1, then
1. The reflected rays are,
2. The incident rays are,
3. What is the angle of incident corresponding to the ray BC?
4. What is the angle of reflection corresponding to the ray CD
18. Rajan was playing with the mirror images of a clock. He looked at the clock in his room. It was showing 1 hour 40 minutes. Draw the position of the hands on the real clock and on its mirror reflection. Write below the picture what time each picture is showing.
19. What is reflection of light?
20. If a ray of light is falling on a plane mirror at an angle of 50 degree is formed, what will be the angle of reflection?
21. What do you mean by lateral inversion?
22. How do you obtain a spectrum of light?
23. Why do we see white color in Newton's disc, when we rotate it very fast?
24. What is a shadow? What things are necessary for the formation of a shadow?
Exercise F. Answer the following questions in detail
1. What are regular and irregular reflection? Explain with the help of diagrams
2. What are the difference between luminous and non-luminous objects? Give two examples of each.
3. Write about two everyday situations that tell you that light travels in a straight line.
4. Differentiate between a reflection and a shadow
5. What are the characteristics of an image formed in a plane mirror?
6. Describe the pictures.
7. Define the following terms
a. Incident ray
b. Reflected ray
c. Normal
d. Angle of incidence
8. Compare the images formed by plane mirror with that by pinhole camera
ICT CORNER
LIGHT
Explore the activity to know about law of reflection.
PROCEDURE:
Step 1: Type the following URL in the browser or scan the QR code from your mobile to open the activity page.
Step 2: Click the torch to see how the light ray reflects off the mirror and drag the torch to different angles. You can learn the angle of incidence at that point will be equal to angle of reflection.
Step 3: Next Scroll down and select the prism. Click the torch how the prism changes the path of the light ray.
Light URL: https://simpop.org/reflection/reflection.htm
📝 Summary
- Light is a type of energy that helps us to see all the things around us and can be detected by the human eye, making it essential for vision.
- There are two main types of light sources: natural sources (like the Sun, stars, and bioluminescent organisms such as fireflies and jellyfish) and artificial sources (like candles, incandescent lamps, neon lamps, and sodium lamps).
- The Sun is the primary and major source of natural light, while the Moon provides light by reflecting sunlight rather than producing its own light.
- Artificial light sources can be categorized into incandescent sources (objects heated to high temperature like candles) and gas discharge sources (electricity passed through gases at low pressure like neon lamps).
- Light has fundamental properties including rectilinear propagation, reflection, speed, interaction with matter, formation of shadows, plane mirror image formation, and spectrum dispersion.