Class 8 - Physics - Light
Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Light enables us to see. In dark room, there is no light for reflection to take place; hence we can’t see.
We will be seeing the objects outside the room only if there is presence of light.
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
In diffused reflection, the surface is rough. Laws of reflection holds true. Due to irregularities on surface, reflected rays scatter in different directions.
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
State the laws of reflection.
Laws of reflection:
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.
Fill in the blanks in the following:
(a) A person 1 m in front of a plane mirror seems to be _______________ m 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.
(a) A person 1 m in front of a plane mirror seems to be 2 m 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: Angle of incidence is equal to the angle of reflection
(a) Always (b) Sometimes
(c) Under special conditions (d) Never
Correct option is :- (a) Always, Angle of incidence is equal to the angle of reflection.
Choose the correct option: 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.
Correct option is: - Image formed by a plane mirror is (b) virtual, behind the mirror and of the same size as the object.
Describe the construction of a kaleidoscope.
Construction of a kaleidoscope is as follows:-
Draw a labelled sketch of the human eye.
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 advice?
Teacher told not to do so as the intensity of laser beam is very high. Laser beam are very
harmful to our eyes which can cause permanent damage to our eyes. So we should not look through the laser beam directly or indirectly.
Explain how you can take care of your eyes.
Following are the points we should keep in mind in order to take good care of eyes:-
What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Angle of incidence = Angle of reflection
angle i = angle r
But given angle i = angle r
angle i = angle i
2(angle i )= 900
(angle i )=(900 /2)
How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Infinite images are formed when two plane mirrors are placed parallel to each other, irrespective of the distance.
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig.
Draw the reflected ray from the second mirror.
(angle i =300 )
Therefore (angle r = 300)
(angle i )=(900- 300) = 600
Therefore (angle r ) = 600
Boojho stands at A just on the side of a plane mirror as shown in Fig. Can he see himself in the mirror?
Also can he see the image of objects situated at P, Q and R?
Boojho cannot see his image because reflected ray won’t reach his eyes.
He can see the image of objects situated at P, Q because the rays coming from P and Q get reflected by mirror and reaches his eyes.
Boojho can’t see image of objects situated at R because the ray from object R does not get reflected.
(a) Find out the position of the image of an object situated at A in the plane mirror Fig.
(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?