# NCERT Physics Class 12 Exemplar Ch 9 Ray Optics and Optical Instruments Part 3

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Q 26. A jar of height h is filled with a transparent liquid of refractive index (Fig.). At the centre of the jar on the bottom surface is a dot. Find the minimum diameter of a disc, such that when placed on the top surface symmetrically about the centre, the dot is invisible.

Ans:

Let d be the diameter of the disc. The spot shall be invisible if the incident rays from the dot at O to the surface at 2dare at the critical angle.

Let

*i*be the angle of incidence.

Then

Now,

Q 27. A myopic adult has a far point at 0.1 m. His power of accommodation is 4 diopters. (i) What power lenses are required to see distant objects?

(ii) What is his near point without glasses?

(iii) What is his near point with glasses? (Take the image distance from the lens of the eye to the retina to be 2 cm.)

Ans:

Let the power at the far point be for the normal relaxed eye.

Then

With the corrective lens the object distance at the far point is The power required is

The effective power of the relaxed eye with glasses is the sum of the eye and that of the glasses.

His power of accommodation is 4 diopters for the normal eye. Let the power of the normal eye for near vision be

Then .

Let his near point be then

.

(iii) With glasses