# CLASS 8 SCIENCE CHAPTER – 16 LIGHT

Question 1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Solution : We cannot see objects in the dark room. But, we can see objects outside the room, in case there is a light outside the room. To see an object need light which must reflect from the object to the eyes.

Question 2. Write down the difference between diffused and regular reflection. Have the laws of reflection failed in the case of diffused reflection?

Solution :

Question 3. Write along side the following statements and give reason to whether the reflection is regular or a diffused reflection and what will appear when light touches the surface.
a. Surface of the Cardboard –
b. Paper Pieces-
c. Polish wooden table –
d. Mirror –
e. Chalk powder –
f. Ceramic floor which has been mopped recently –
Solution :
(a) Cardboard surface- Diffused reflection
The surface of the cardboard is a kind of irregular surface. Hence, diffused reflection will take place from a cardboard surface.

(b) Bits of paper- Diffused reflection
Although a piece of paper may look smooth, but it has many irregularities on its surface. Because of this reason, it will give a diffused reflection.

(c) Wooden table that has been polished-Regular reflection
Surface that has been recently polished can be a good example of smooth surface. Wooden table that has been polished has surface that is smooth.

(d) Mirror- Regular reflection
A mirror has a very smooth surface. That is why it gives a regular reflection.

(e) 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, diffused reflection will appear from chalk powder.

(f) Ceramic floor which has been mopped recently- Regular reflection
The ceramic floor which has been recently mopped can be a good example of surface that is regular. Because water makes the ceramic glossy. Thus, thereflections that are regular takes occurs on this surface.

Question 4. What is “law of reflection”?
Solution : The law of reflection states that:
(a) The angle of reflection and the angle of incidence both arealways equal to one another.
(b) The reflected ray,the incident ray, and the normal to the reflective surface at the point of incidence all comes on the same plane.

Question 5. Elaborate experiment to justify that the reflected ray, the incident ray, and the normal ray lies in the same plane at the point of incidence.
Solution : 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 if 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.

Question 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 this 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.

Solution :
(a) A person 1 m in front of a plane mirror seems to be2m away from this 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.

Question 7.The angle of reflection and the angle of incidence are equal.
a) Sometimes b) always c) never d) Under special conditions
Solution : (b) The angle of incidence is always equal to the angle of reflection. This is the first law of reflection.

Question 8. An image formed by the plane mirror is
a. Virtual, behind the mirror and of the same size as the object.
b. Real, behind the mirror and of the same size as the object.
c. Virtual, behind the mirror and enlarged
d. Real at the surface of the mirror and enlarged.
Solution : (a) A virtual image is formed by the plane mirror, behind the mirror and of the same size as the object. The plane mirror formed an image is of the same size as the object. The image is formed behind the mirror. On a screen the image cannot be obtained and hence, it is a virtual image.

Question 9.Elaborate the construction of a kaleidoscope:
Solution :
The construction of a kaleidoscope: Take three rectangular mirror strips of dimensions 15cm x 4cm (l x b) are joined together to form a prism as shown in (fig a). below. A prism is fixed into a circular cardboard tube. The circular cardboard tube should be slightly longer than the prism (fig b). This circular tube is now closed at one end with a cardboard disc. This disc has a hole in it through which we can see (fig c). 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 colored glass are placed. This end is now closed by a round glass plate allowing enough space for the colored glass pieces to move (fig d).

Question 10. Draw a labeled sketch of the human eye.
Solution :

The labelled sketch of the human eye is shown below:

Question 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 teachers advise?
Solution : The laser beam is harmful for the human eyes, since its intensity is very high. Hence, it can cause damage and lead to blindness. This is the reason one should not look at laser beam directly or indirectly.

Question 12. Explain how you can take care of your eyes.
Solution :
Following are few points that can be taken into account to take care of your eyes:
(i) If there is any problem in eyes, one should consult an eye specialist.
(ii) Try to avoid reading in dim light and very bright light.
(iii) Never look at the sun or any powerful light source directly.
(iv) Do not rub your eyes.
(v) If any particle enters your eyes, wash your eyes with clean water.
(vi) Keep an average of atleast 25 cm between book and eyes while reading.

Question 13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Solution :
We know that angle of incident ray is equal to angle of reflected ray i.e. ∠i = ∠r
Here, it is given that ∠i + ∠r = 90°
⇒ ∠i + ∠i = 90°
⇒ 2∠i = 90°
⇒ ∠i = 45°

Question 14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Solution : Infinite images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm.

Question 15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30 degree as shown in fig. 16.19. Draw the reflected ray from the second mirror.

Solution :

Question 16. Boojho stands at A just on the side of a plane mirror 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?

Solution :

Question 17. (a) Find out the position of the image of an object situated at A in the mirror.
(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?

Solution :
(a) A image will be formed at the same distance behind the mirror.
(b) Yes.
(c) Yes.
(d) The image remains at the same place. The image moves only if the object or the mirror is moved.

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