# NCERT Physics Class 12 Exemplar Ch 10 Wave Optics Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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**Q 22** The optical properties of a medium are governed by the relative permittivity and relative permeability . The refractive index is defined as for ordinary material *r* and and the positive sign is taken for the square root. In 1964, a Russian scientist V. Vassalage postulated the existence of material with . Since then such ‘met materials’ have been produced in the laboratories and their optical properties studied. For such materials as light enters a medium of such refractive index the phases travel away from the direction of propagation.

(i) According to the description above show that if rays of light enter such a medium from air (refractive index ) at an angle θ in 2^{nd} quadrant, them the refracted beam is in the 3^{rd} quadrant.

(ii) Prove that Snell՚s law holds for such a medium.

Ans:

- Suppose the postulate is true, then two parallel rays would proceed as shown in Fig. 1. Assuming ED shows a wave front then all points on this must have the same phase. All points with the same optical path length must have the same phase.

Thus

Or BC =

As showing that the postulate is reasonable. If however, the light proceeded in the sense it does for ordinary material (viz. in the fourth quadrant, Fig. 2)

Then —

Or,

As

Showing that this is not possible. Hence the postalate is correct.

- From Fig. 1.

and

Since