(a) State the laws of refraction of light. Give an expression to relate the absolute refractive index of a medium with speed of light in vacuum. (b) The refractive indices of water and glass with respect to air are 4/3 and 3/2 respectively. If the speed of light in glass is 2x10⁸ms⁻¹, find the speed of light in (i) air, (ii) water.

(a) Law of Refraction : (1) The ratio of sine of angle of incidence to the sine of angle of refraction for a particular pair of media is constant. Therefore, if angle of incidence = i, angle of refraction = r, sin i / sin r = constant. The value of this constant depends upon temperature and wavelength of light. This is also known as Snell's law (2) The incident ray, the refracted ray and the normal at the point of incidence, all the lie in the same plane. Refractive index (of a medium) = speed of light in air (or vaccum) / speed of light in medium ⇒ μm = c / vm (b) μglass = c / vglass ⇒ 3/2 = c / 2 × 10⁸ m/s (i) ⇒ c = 3 × 10⁸ m/s (ii) μwater = c / vwater ⇒ vwater = (3 × 10⁸ m/s) / (4/3) = 9/4 × 10⁸ m/s