Page 94 - Physics - XII

Q20. Will a colourless slab be visible if immersed in a transparent liquid of the same refractive
index as that of the slab? State the reason for it. (NCERT)
Because the refractive index of both glass and colourless liquid is same, so light rays will not face
any deviation in their path while crossing the interface. Also, as glass is transparent there will not be
any refraction as well.
As light faces no deviation at all, it will pass straight through the glass slab and, hence glass slab will
be invisible.

Q21. You have three slabs of same dimensions – the fi rst one being hollow and completely fi lled with
water, the second one is made of crown glass and the third fl int glass. If each of them has a
coloured mark at the bottom, in which case will it appear to have risen the most?
Given its n > n > n (NCERT)
fl int crown water
Ans. The coloured mark at the bottom will appear to be raised the most in the case of the glass slab
containing water.
Q22. What will happen if you are given a convex lens of small focal length? (NCERT)
Ans. A convex lens of small focal length will be thick. So, the distance measured from the surface of
convex lens will not be equal to the distance measured from its optical centre. Hence, the result
will vary.

Q23. What are the basic assumptions you have made in performing this experiment? (NCERT)
Ans. The basic assumptions made in performing the experiment are as follows:
(i) The convex lens must be thin.
(ii) Both surfaces must have the same radius of curvatures.
(iii) The plane mirror is perfectly horizontal.
Q24. Why do we have to raise the object pin upward after fi lling water between the lens and the
mirror? (NCERT)

Ans. Water and convex lens will work as a combined lens and, hence the image is formed at larger
distance.

COMPETENCY BASED QUESTIONS

Multiple Choice Questions

Q1. The image of the needle placed 45 cm from a lens is formed on a screen placed 90 cm on the
other side of the lens. The displacement of the image if the needle is moved by 5.0 cm away
from the lens is

(a) 10 cm, towards the lens (b) 15 cm, away from the lens
(c) 15 cm, towards the lens (d) 10 cm, away from the lens

Q2. A convergent beam of light passes through a diverging lens of focal length 0.2 m and comes to
focus 0.3 m behind the lens. The position of the point at which the beam would converge in the
absence of the lens is
(a) 0.12 m (b) 0.6 m (c) 0.3 m (d) 0.15 m

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