Wednesday, 8 January 2020

Light

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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.

Answer:
When we are in a dark room then we cannot see objects in the room. We can see the objects outside the room, because out of the room the light is available and the rays of light can enter our eyes after reflection from the objects.
Question 2.
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Answer:
Regular Reflection
Diffused Reflection
(i) All the reflected rays are parallel.
(i) The reflected rays are not parallel.
(ii) It occurs on a smooth and polished surface.
(ii) It occurs on the rough surface.
(iii) Reflected rays are in one direction.
(iii) Reflected rays are scattered in different directions.
No, diffuse reflection doesn’t mean the failure of laws of reflection.
Question 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.
1.     Polished wooden table
2.     Chalk powder
3.     Cardboard surface
4.     Marble floor with water spread over it
5.     Mirror
6.     Piece of paper
Answer:
1.     Regular reflection will take place because the surface is plane and polished.
2.     Diffused reflection will take place because the surface is rough.
3.     Diffused reflection will take place because the surface is rough.
4.     Regular reflection will take place because the surface is smooth and plane.
5.     Regular reflection will take place because the surface is plane and polished.
6.     Diffused reflection will take place because the surface is rough.
Question 4.
State the laws of reflection.

Answer:
The laws of reflections are:
  • The incident ray, the normal and the reflected ray, all lie in the same plane.
  • The angle of incidence is equal to the angle of reflection.
Question 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.

Answer:
Activity: To show that the incident ray, reflected ray and the normal at the point of incidence lie in the same plane.
Materials Required: Plane mirror, holder, ray box, etc.
Procedure: Fix sheet of white paper, a little beyond the edge of the board. Place a plane mirror strip vertically to the paper using a stand. Throw light from a ray box on the mirror. Look at the reflected ray. Mark the incident ray, normal ray and reflected ray. Fold the paper which is beyond the edge of the board. You will observe that the reflected ray is not seen in the folded portion of the chart paper. Now bring the folded portion back to its original position. The reflected ray of light is again seen on the page.
Conclusion: The sheet on the board can be considered as a plane. The incident ray, the reflected ray, the normal at the point of incidence lie in the same plane.

Incident ray, reflected ray and normal 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 his image.
(b) If you touch your ______ ear with a 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.


Answer:
(a) 2
(b) left, left hand
(c) larger
(d) lesser
Choose the correct option in Questions 7-8.
Question 7.
The angle of incidence is equal to the angle of reflection
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never

Answer:
(a) Always
Question 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.

Answer:
(b) virtual, behind the mirror and of the same size as the object.
Question 9.
Describe the construction of a kaleidoscope.

Answer:
Kaleidoscope is a device based on the principle of multiple reflections. It consists of three long and narrow strips of plane mirrors inclined at an angle of 60° to one another forming prism. This is fitted in a tube. One end of this tube is closed by a cardboard disc having a hole at its centre. To the other end touching the mirrors plane glass plate is fixed on which broken pieces of coloured bangles are placed. This end of the tube is closed by a ground glass plate.

Question 10.
Draw a labelled sketch of the human eye.

Answer:
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 teacher’s advise?

Answer:
Teacher has advised Gurmit not to do so because laser light is very harmful for her eyes and can cause a permanent defect in the eye. Person can even lose his or her eyesight if laser torch is directed over the eyes.
Question 12.
Explain how you can take care of your eyes.

Answer:
Eyes are very precious. We must take proper care of them. We must
  • always sit straight while reading or writing.
  • if advised, use suitable spectacles.
  • wash our eyes with clean water frequently.
  • not look at the sun directly.
  • always read or write in a proper light.
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?

Answer:
Here, the angle of reflection is 90°. As we know, according to the laws of reflection that angle of incidence is equal to angle of reflection.
Here, the angle between the incident ray and reflected ray is 90°.
i.e.,
i + r = 90°
Since,
i = r
We can write,
i + i = 90°
2i = 90°
i = 45°
Angle of incidence = 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?

Answer:
Here, mirrors are placed parallel to each other 40 cm apart. Therefore, the infinite number of images will be formed.
Question 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.


Answer:
Question 16.
Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.21. Can he see himself in the mirror? Also, can he see the image of objects situated at P, Q, and R?


Answer:
No, Boojho can’t see himself in the mirror. He can see the image of the object at P and Q but not of R.

Question 17.
(a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.23).
(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?


Answer:
(a) It is shown in the following figure.

(b) Yes, Paheli can see the image of A.
(c) Yes, Boojho can see the image of A.
(d) Image of the object at A will not move as an object is not moving.