Physics Class 12 NCERT Solutions: Chapter 9 Ray Optics and Optical Instruments Part 11 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Snell՚S Law of Refraction

Q: 22. At what angle should a ray of light be incident on the face of a prism of refracting angle so that it just suffers total internal reflection at the other face? The refractive index of the material of the prism is .


The incident, refracted, and emergent rays associated with a glass prism ABC are shown in the given figure.

Q_22_The Incident, Refracted, and Emergent Rays Associated W …

Angle of prism,

Refractive index of the prism,

Incident angle at face AB

Refracted angle

Angle of incidence at the face

Emergent angle

According to Snell՚s law, for face , we can have:

It is clear from the figure that angle

According to Snell՚s law, we have the relation:

Hence, the angle of incidence is .

Q: 23. You are given prisms made of crown glass and flint glass with a wide variety of angles. Suggest a combination of prisms which will:

(A) Deviate a pencil of white light without much dispersion,

(B) Disperse (and displace) a pencil of white light without much deviation.


(A) Place the two prisms beside each other. Make sure that their bases are on the opposite sides of the incident white light, with their faces touching each other. When the white light is incident on the first prism, it will get dispersed. When this dispersed light is incident on the second prism, it will recombine and white light will emerge from the combination of the two prisms.

(B) Take the system of the two prisms as suggested in answer (a) . Adjust (increase) the angle of the flint-glass-prism so that the deviations due to the combination of the prisms become equal. This combination will disperse the pencil of white light without much deviation.

Q: 24. For a normal eye, the far point is at infinity and the near point of distinct vision is about in front of the eye. The cornea of the eye provides a converging power of about dioptres, and the least converging power of the eye-lens behind the cornea is about dioptres. From this rough data estimate the range of accommodation (i.e.. , the range of converging power of the eye-lens) of a normal eye.


Least distance of distinct vision,

Far point of a normal eye,

Converging power of the cornea,

Least converging power of the eye-lens,

To see the objects at infinity, the eye uses its least converging power.

Power of the eye-lens,

Power of the eye-lens is given as:

To focus an object at the near point, object distance .

Focal length of the eye-lens = Distance between the cornea and the retina = Image distance

Hence, image distance,

According to the lens formula, we can write:


Power of the eye-lens,

Hence, the range of accommodation of the eye-lens is from .