The Light, Class 8th Notes

THESE NOTES ARE PREPARED ACCORDING TO JKBOSE SYLLABI IN SIMPLIFIED AND CONCISE  PATTERN

Q. Suppose you are in the dark room. Can you see objects in the room. Can you see objects outside the room? Explain?

Ans: To see any object, we need source of light and eyes.

Inside the dark room, there is no source of light, Therefore we cannot see anything.

If we move out of the dark room, we can see objects around us.

Q. Differentiate between regular and diffused reflection? Does diffused reflection mean failure of the laws of reflection?

Ans:

1. Regular reflection occurs on plane polished surfaces whereas diffused reflection occurs on uneven surfaces

2. Regular reflection follows laws of reflection, Diffused reflection does not follow laws of reflection.

The laws of reflection are valid on plain polished surfaces, The diffused reflection occurs on such surfaces which are not polished. Hence it is not failure of laws of reflection.


Q. Mention against each whether regular or diffused reflection will take place. Justify your answer.

A. Polished Wooden table: Regular reflection.

B. Chalk powder: Diffused reflection.

C. Cardboard: Diffused reflection.

D. Marble floor with water: Regular reflection

E. Mirror: Regular reflection

F. Piece of Paper: Diffused reflection.


Q. State the laws of reflection?

Ans: The bouncing back of light when it touches any surface is called reflection of light.
There are two laws of reflection:

1. The Incident ray, The reflected ray and the normal are all in same plane.

2. The angle of Incidence is equall to angle of reflection.

Q. Describe the activity to show incident ray, reflected ray and normal are in same plane?

Ans: Take a torch, Mirror strip and adjust it on sheet of paper,

Put the light from torch on mirror strip at an angle such that the path is visible in form of incident ray and reflected ray.

Inbetween the incident ray and reflected ray,  Cut the sheet of paper and fold it upwards.

We will not see the path of light ray visible on folded part.

This activity demonstrates that incident ray, Reflected ray and normal are all in same plane.


Q. Fill in the Blanks

  (a) A person 1 m in front of a plane mirror seems to be ___2____m 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__RIGHT_____.

 (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.

Q. Angle of incidence is equal to the angle of reflection.

Ans:  (a) Always

Q. Image formed by a plane mirror is:

   Ans. (b) Virtual, behind the mirror and of the same size as the object.


Q.  Describe the construction of a kaleidoscope.

Ans. 
Take three rectangular mirror strips, join them together in the shape of prism.

Put it  in circular cardboard tube. The tube should be longer than the mirror strips.

Close one end of the tube by disc having a hole in the centre.

At the other end, touching the mirrors, fix a circular glass plate.

Put pieces of coloured bangles inside it.

Thus our kaleidoscope is formed.

Q. Draw the labelled structure of human eye?

Ans:

Q..   Explain how you can take care of your eyes.

Ans: Eyes can be taken care of by following ways.

 1. Do not read in too little or too much light.

  2. Wash your eyes with cold water.

 3. Do not look at the sun directly.

 4. Always read at the normal distance for vision.

 5. Do not rub your eyes with dirty hand.

Q. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Ans. The angle of incidence ray will be 45°.


Q.   How many images of a candle will be formed if it is placed between two parallel mirrors separated by 40 cm?

Ans. The number of images formef will be infinite.


Q.  Two mirrors meet at right angles. A ray of light is incident at an angle of 30° as shown in Fig. 16.10. Draw the reflected ray from the second mirror.

Ans.

The reflected ray on 2nd mirror will be 60 degree.

Q. Yasir stands atA just on the side of a plane mirror as shown in Fig. 16.11. Can he see himself in the mirror? Also can. he see the image of objects situated at P, Q and R?

Ans: Simce yasir is away from mirror, he cannot see himself.

He can see images at P, Q and R


Q. (a) Find out the position of the image of an object situated at A in the plane mirror

  (b) Can saba at B see the image?

   (c) Can yasir at C see this image?

     (d) When saba moves from B to C, where dries the image of A move?

Ans:

A. The image of object at position A is straight behind the mirror as far as A is infront of mirror

B.   Yes, saba can see the image at B.

C.  Yes, yasir can see the image.

 D. When saba moves from B to C, the image A may seem to be larger and at lesser distance.





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