and . The refractive index of glass with respect to air  will be equal to

a)

b)

c)

d)

(b)

In the case of refraction if  is the refracted wave front and  and  are the speed of light in the two media, then in the time the wavelets from  reaches , the wavelet from  will reach , such that

But in           …(ii)

While in    …(iii)

From equations (i), (ii) and (iii)

Also