Light
EXERCISES
BE PROMPT
A. Fill in the blanks.
1. We are able to see objects which are not in direct light due to scattering of light.
2. A beam of light can be parallel, convergent and divergent.
3. Light is a form of energy.
4. Lunar eclipse occurs when the Earth casts its shadow on the Moon.
5. A shadow is always formed on the opposite side of the light source.
6. Solar eclipse occurs on a new moon day.
B. State whether the following statements are true (T) or false (F). Correct the false statements.
1. Opaque objects casts a dark shadow. [T]
2. The sun and stars are artificial luminous bodies. [F]
= The sun and stars are natural luminous bodies.
3. Light travels in a straight line. [T]
4. Lunar eclipse occurs on a full moon night. [T]
5. A point source forms a smaller umbra and a larger penumbra region, thus giving a fuzzy shadow. [F]
= A broad source forms a smaller umbra and a larger penumbra region, thus giving a fuzzy shadow.
6. A pinhole camera forms an erect image. [F]
= A pinhole camera forms an inverted image.
C. Choose the correct option.
1. Which of the following is an artificial luminous object?
(a) Torch. [✓]
(b) Moon. [ ]
(c) Tree. [ ]
(d) Chair. [ ]
2. Which of the following is a translucent object?
(a) Grease paper. [✓]
(b) Glass. [ ]
(c) Air. [ ]
(d) Marble. [ ]
3. Which of the following produces a parallel beam of light?
(a) Lighthouse. [ ]
(b) Bulb. [ ]
(c) Sun. [✓]
(d) Candle. [ ]
4. Which of the following is not a characteristic of shadow?
(a) It is always formed on the opposite side of the light source. [ ]
(b) It is of the same size as that of the object. [✓]
(c) It is always dark in colour. [ ]
(d) It is of the same shape as the object. [ ]
5. The size of the shadow depends on ________
(a) the distance between the source of light and the object. [ ]
(b) the distance between the object and the screen. [ ]
(c) the size of the source of light and object. [ ]
(d) all of these. [ ]
6. The phenomenon showing rectilinear propagation of light is/are __________
(a) formation of shadows. [ ]
(b) occurrence of eclipses. [ ]
(c) both of these. [✓]
(d) none of these. [ ]
D. Match the columns.
=
|
Column A |
Column B |
|---|---|
|
1. Moon |
(a) Transparent object. [2] |
|
2. Glass |
(b) A number of rays of light. [4] |
|
3. Bulb |
(c) Real image. [5] |
|
4. Beam |
(d) Non-luminous object. [1] |
|
5. Pinhole camera |
(e) Artificial source of light. [3] |
SHORT AND PRECISE
A. Complete the following flowchart using the words given in the box.
=
[eclipses, formation, transparent objects, solar eclipse, stars, natural, candle, lunar eclipse, opaque objects, luminous bodies, tube light, non-luminous bodies, rectilinear propagation of light]
B. Give reasons for the following statements.
1. Planets are said to be non-luminous bodies.
= Planets are said to be non-luminous bodies because they have no light of their own. They shine because they reflect the light falling on them.
2. A glass plate does not form a shadow.
= A glass plate does not form a shadow because it is a transparent object. Light can pass completely through it.
3. Light follows straight line path in air.
= Light follows straight line path in air because of its rectilinear propagation.
4. The visibility through a wax paper is poor.
= The visibility through a wax paper is poor because it is a translucent material. It allows light to pass partially through it.
5. A pinhole camera forms real and inverted image.
= A pinhole camera forms real and inverted image because the light rays intersect and cross over while passing through the pinhole.
C. Answer in short.
1. Why is moon considered to be a non-luminous body?
= The moon is considered to be a non-luminous body because it does not have light of its own. It reflects the light of the sun and appears to be shining in the night sky. So moonlight is actually solar light reflected by the moon.
2. Give uses of transparent, translucent and opaque objects.
= Uses of transparent objects : Transparent objects are used in making things such as showcases, windowpanes, doors, spectacles, binoculars, telescopes and microscopes.
Use of translucent objects : Translucent objects are used in making doors and windowpanes of bathrooms, clinics and offices, laboratory equipments, etc.
Use of opaque objects : Opaque objects are used in making things where we do not want light to pass through.
3. What is the nature of the beam of light produced by a lighthouse?
= The light emitted by a lighthouse is an example of divergent light beams. The rays of light emitted by a small (point) source which travel in different directions from a divergent beam of light.
4. How are shadows formed?
= Shadows are formed when the path of light is obstructed by an opaque object. The formation of shadow is direct consequence of the rectilinear propagation of light. This is because an opaque substance does not allow light to pass through it.
5. What happens to the size of umbra and penumbra when the distance between the source, the object and the screen is changed?
= A larger and fuzzier shadow is formed, if the object is placed closer to the light source. If the object is closer to the screen, penumbra region will be smaller and a well-defined shadow will be formed. Also, if the screen is moved farther from the object-source arrangement, the width of umbra reduces. This results in a large penumbra region and thus a faint shadow.
6. What do you mean by an eclipse? How many types of eclipses are formed?
= During their course of revolution, sometimes the sun, the earth and the moon come in a straight line. When this happens, the heavenly body in the centre (either the earth of the moon) blocks the sunlight thereby casting its shadow on the last body. Thus, we can define an eclipse as the partial or total blocking of the light of the sun by a heavenly body.
AT LENGTH
A. Explain the following terms.
1. Ray of light.
= Light travels along a straight line path and is depicted as ray.
2. Beam of light.
= When a number of rays travel together in one direction, they are collectively known as a beam of light.
3. Umbra.
= Umbra is the darker region of the shadow formed on the screen.
4. Penumbra.
= Penumbra is the grey zone surrounding the umbra which receives light partially.
5. Rectilinear propagation of light.
= Light always travels in a straight line in a medium. This property of light travelling in a straight line is called rectilinear propagation of light.
B. Differentiate between the following.
1. Luminous objects and non-luminous objects.
=
|
Luminous Objects |
Non-luminous Objects |
|---|---|
|
1. They have their own light. |
1. They do not have their own light. |
|
2. Examples – Sun, stars, candle etc. |
2. Examples – Chair, table, tree etc. |
2. Solar eclipse and lunar eclipse.
=
|
Solar Eclipse |
Lunar Eclipse |
|---|---|
|
1. It happens in new moon. |
1. It happens in full moon. |
|
2. The moon comes between sun and earth. |
2. The earth comes between the sun and the moon. |
|
3. It lasts for a few minutes at any given place. |
3. It lasts for few hours at any given place. |
|
4. It can only be viewed from a small area of the earth. |
4. It can be viewed from anywhere on the night side of the earth. |
3. Broad source and point source of light by giving diagrams of shadows formed by them.
= When an opaque object is placed in the path of a point source of light, the shadow formed on the screen is uniformly dark, sharp and is of the same shape. This darker region of the shadow formed on the screen is known as umbra.
Now, if we replace the point of source with a broad source of light, we will notice that the shadow formed will now have two regions. One is the darker inner region, which is devoid of any light. This is the umbra. The grey zone surrounding the umbra, which receives light partially is called penumbra.
4. Transparent objects and opaque objects.
=
|
Transparent Object |
Opaque Object |
|---|---|
|
1. Light can completely pass through it. |
1. Light can not pass through it. |
|
2. An object is clearly visible when viewed through a fully
transparent medium. |
2. An opaque object always casts its shadow when kept in the path of
light. |
|
3. Transparent objects are used in making things such as showcases,
windowpanes, doors, spectacles, binoculars, telescopes and microscopes. |
3. Opaque objects are used in making things where we do not want light
to pass through. |
C. Answer in details.
1. Explain the scattering of light.
= Air consists of various gases and dust particles. Light always travels in a straight line path. When light passes through air, some of it is absorbed by these particles and re-emitted in different directions. The deflection of light from its straight path by particles of a medium such that some of the light is absorbed and re-emitted in different directions is called scattering of light.
It is due to scattering of light that we are able to see objects which are not in direct path of sunlight. Our atmosphere appears bright because of scattering of sunlight and this is the reason we can see objects around us at dusk and dawn time. Similarly, if we light a candle or a torch in a dark room, the dust particles scatter light in all directions and we are able to see even things which do not lie directly in the straight-line path of the candle/torch.
2. Differentiate between transparent, translucent and opaque objects on the basis of the amount of light they allow to pass through and shadow formed by them.
= Transparent objects let light completely pass through it. Translucent objects let light partially pass through it. Opaque objects do not let light pass through it.
Transparent objects do not form any shadow. Translucent objects form hazy shadow. Opaque objects from dark shadow.
3. Give one experiment that confirms that light travels in a straight line.
= Aim : To prove that light travels in a straight line.
Materials required : Three sheets of cardboards of same size, plasticine or clay and candle.
Procedure : Take the three cardboards and make a hole in the centre of each. Make sure that the size and position of the hole is same in each cardboard.
Place the cardboards in vertical position on a table with the help of clay or plasticine in such a way that the holes in them are in straight line.
Light the candle and place it on the table in front of the cardboards as shown in the figure.
Look through the hole of the last cardboard and observe.
Now displace any one of the cardboards slightly and look through the hole of the last cardboard again and observe.
Observation :
The flame of the candle can be seen clearly through the holes when all the three holes on cardboards are in straight line.
You can not see the flame is anyone of the cardboards are displaced.
Conclusion :
Light always travels in a straight line.
4. Define a beam of light and explain its types with the aid of diagrams.
= Light travels along a straight line path and is depicted as a ray. It is a form of energy.
A light ray is diagrammatically represented by a straight line with an arrowhead. The arrowhead indicates the direction in which the ray moves.
When a number of rays travel together in one direction, they are collectively known as a beam of light. A torch produces a beam of light. A beam of light can be parallel, convergent and divergent.
Parallel beam : When a number of rays move parallel to each other, the beam is called a parallel beam of light. It is produced by a distant source of light like the sun.
Convergent beam : A collection of rays of light coming from different directions which converge or meet at a single point is called a convergent beam of light. Sunlight passing through a magnifying lens is an example of convergent beam.
Divergent beam : The rays of light emitted by a small (point) source which travel in different directions from a divergent beam of light. Light emitted by a bulb, an LED, a candle or a lighthouse are examples of divergent beams.
5. Write a note on the formation of shadows.
= A shadow is a dark path formed when the path of light is obstructed by an opaque object. The formation of shadow is a direct consequence of the rectilinear propagation of light. This is because an opaque substance does not allow light to pass through it.
An opaque substance casts a dark shadow. A translucent material like a butter paper blocks the path of the light only partially and hence, produces a faint shadow. A transparent material like glass allows all the light to pass through, and does not cast any shadow.
To from a shadow the following things are required.
A source of light.
An opaque object.
A screen.
6. What is a solar eclipse? Explain with the help of a neat diagram.
= A solar eclipse occur when the moon during its course of revolution passes between the sun and the earth and the three heavenly bodies position themselves in a straight line. The moon fully or partially covers the sun, as viewed from some parts of the earth. Since the light from the sun is blocked by the moon, the shadow of the moon falls on the earth and it becomes dark during the daytime. Solar eclipse occurs only on a new moon day.
7. What happens during a lunar eclipse? Explain the types of lunar eclipses with the help of a neat diagram.
= The moon itself produces no light and is seen only when light from the sun falls on it and gets reflected. A lunar eclipse occurs when the earth comes in between the sun and the moon and its shadow falls on the moon. For people on the earth, the moon is not visible for some time and remains hidden. This natural phenomenon is known as the lunar eclipse.
8. Explain how an image is formed by a pinhole camera. Mention the characteristics of the image formed.
= The formation of an image by a pinhole camera can be understood by a simple ray diagram. Consider an object XY kept in front of the pinhole O of the camera. Although light rays from the object travel in all directions, we will only take those rays into consideration that pass through the pinhole and thus contribute to image formation.
A ray of light coming from point X travels along XO, passes through the pinhole O and finally falls on the screen at X'. Similarly, a light ray coming from point Y travels along YO, passes through the pinhole O and falls on the screen at Y'. All the other rays which start between X' and Y'. While passing through the pinhole, the light rays intersect and cross over. Thus, the final image Y'X' is an inverted image of the object XY.
9. Write the advantage and disadvantages of a pinhole camera.
= Advantages of Pinhole Camera :
❐ There is no need of focussing.
❐ Since there is no lens used in a pinhole camera, the image obtained is free from optical defects like colour distortion.
Disadvantage of Pinhole Camera :
❐ The image is obtained on a screen and no permanent record of the image can be maintained.
❐ The image formed lacks sharpness and is faint. No details can be observed in it.
❐ An image of a moving object cannot be captured using a pinhole camera. It only forms images of stationary objects.