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NCERT Solutions for Class 8 Science chapter 16 – Light
Back Exercise
1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain.
Soln:
If a person is inside the room where there is no light, it is then impossible to visualize the object inside the room but the object out of the room can be seen easily.
When light falls on eyes after reflecting from the object, it becomes visible. If the room is dark, then the object which is in the room reflects no light. Hence, the person is not able to see the objects in the room where there is no light.
2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Soln:
| S.No | Regular Reflection | Diffused Reflection |
| 1. | It occurs when the surface is smooth. | It occurs when the surface is a rough surface. |
| 2. | Reflected rays move in a particular direction. | Reflected rays scattered in random directions. |
| Example: Reflection by the plane mirror | Example: Reflection by the road surface. |
The laws of reflection have not failed because each ray obeys the law of reflection. All the reflected rays are parallel to each other, whereas in diffused reflection the rays aren’t parallel to the incident rays.
3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) Mirror
(f) Piece of paper
Soln:
a) The wooden table that has been a polished-Regular reflection
The surface that has been recently polished can be a good example of a smooth surface. The wooden table that has been polished has a surface that is smooth.
b) White Chalk powder that is used in school- Diffused reflection
Chalk powder spread on a surface is an example of an irregular surface. Hence, it is rough. Therefore, the diffused reflection will appear from chalk powder.
c) Cardboard surface- Diffused reflection
The surface of the cardboard is a kind of irregular surface. Hence, the diffused reflection will take place from a cardboard surface.
d) Marble floor – Regular reflection
Marble floor can be a good example of a surface that is regular. Because water makes the ceramic glossy. Thus, the reflections that are regular occurs on this surface.
e) Mirror- Regular reflection
A mirror has a very smooth surface hence it gives a regular reflection.
f) Piece of paper- Diffused reflection
Although a piece of paper may look smooth, it has many irregularities on its surface. Because of this reason, it will give a diffused reflection.
4. State the laws of reflection.
Soln:
The law of reflection states that
a) The angle of reflection and the angle of incidence both are always equal to one another.
b) The reflected ray, the incident ray, and the normal to the reflective surface at the point of incidence all come on the same plane.
5. Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Soln:
On a table, place a plane mirror perpendicular to the plane of the table. Make a small hole in a paper and hold it perpendicular to the plane of the table. Try to do this experiment in a dark room. Take one more piece of paper and place it on the table so that it makes contact with the mirror. Draw a line perpendicular to the mirror on the piece of paper which is on the table. Now beam light rays with the help of a torch through the small hole such that the beam of light hits the normal at the bottom of the mirror. The ray of light will be reflected in the light rays from the hole are incident on the mirror. Looking at the piece of paper on the table, we can easily show that the incident ray, the normal line and the reflected ray at the point of incidence lie in the same plane.
6. Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be _______________ m away from his image.
(b) If you touch your ____________ ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with ____________.
(c) The size of the pupil becomes ____________ when you see in dim light.
(d) Night birds have ____________ cones than rods in their eyes.
Soln:
(a) A person 1 m in front of a plane mirror seems to be 2m away from his image.
(b) If you touch your left ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with left hand.
(c) The size of the pupil becomes large when you see in dim light.
(d) Night birds have less cones than rods in their eyes.
Choose the correct option in Questions 7 – 8
7. Angle of incidence is equal to the angle of reflection.
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never
Soln:
Answer is (a) Always
8. Image formed by a plane mirror is
(a) virtual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of the same size as the object.
(c) real at the surface of the mirror and enlarged.
(d) real, behind the mirror and of the same size as the object.
Soln:
Answer is (b) virtual, behind the mirror and of the same size as the object.
9. Describe the construction of a kaleidoscope.
Soln:
The construction of a kaleidoscope:
Take three rectangular mirror strips of dimensions 15cm x 4cm (l x b) and join them together to form a prism. A prism is fixed into a circular cardboard tube. The circular cardboard tube should be slightly longer than the prism. This circular tube is now closed at one end with a cardboard disc. This disc has a hole through which we can see. At the other end of the circular tube, a plane glass plate is fixed. It is important that this glass plate touches the prism mirrors. On this glass plate, several small and broken pieces of coloured glass are placed. This end is now closed by a round glass plate allowing enough space for the coloured glass pieces to move.
10. Draw a labelled sketch of the human eye.
Soln:
11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advise?
Soln:
Because of the intensity of the laser light is very high, it is harmful to the human eyes. It can cause damage to the retina and leads to blindness. Hence, it is advisable not to look at a laser beam directly.
12. Explain how you can take care of your eyes.
Soln:
The following points help a person to take care of his eyes:
a) Reading should not be done in bright light as well as in dim light.
b) He should visit an eye specialist on a regular interval of time.
c) If any small insects or dust particles enters his eyes, do not rub them but clean them immediately with cold water.
d) He should avoid direct exposure of sunlight to the eye.
e) While reading, there should be a distance of at least 25 cm between the eyes and the book.
13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Soln:
If the reflected ray is at the angle of 90o to the incident ray, then the angle of incidence is 45o. According to the law of reflection, the angle of incidence and the angle of reflection are equal. Therefore, the angle of incidence and the angle of reflection both are 90/2=45o.
14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Soln:
If a candle is placed between two parallel plane mirror separated by 40 cm, then the multiple and infinite images will be formed due to the multiple reflections between the mirrors. The infinite numbers of images are formed when two mirrors are placed parallel to each other.
15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.
Soln:
The first law of reflection is used to obtain the path of reflected light.
It can be observed that the given ray of light will reflect from the second mirror at an angle 60°.
16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?
17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A move?
Soln:
a) Image of an object placed at A is formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror
b)Yes Paheli at B can see this image.
c) Yes Boojho at C can see this image.
d) Image of the object at A will not move. It will remain at the same position when Paheli moves from B to C.