Question

(a) State two laws of refraction of light.
(b) Define the term absolute refractive index of a medium. A ray of light enters from vacuum to glass of absolute refractive index 1.5. Find the speed of light in glass. The speed of light in vacuum is $3 \times 10^8$ m/s.

Answer 1

(a). The incident ray, refracted ray, and the normal to the surface at the point of incidence lie in the same plane.
The ratio of the sine of the angle of incidence ($i$) to the sine of the angle of refraction ($r$) is constant ($n = \frac{\sin i}{\sin r}$).

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(b). The absolute refractive index ($n$) of a medium is the ratio of the speed of light in vacuum ($c$) to the speed of light in the medium ($v$):
$n = \frac{c}{v}.$
For glass:
$1.5 = \frac{3 \times 10^8}{v} \implies v = \frac{3 \times 10^8}{1.5} = 2 \times 10^8 \, \text{m/s}.$
The speed of light in glass is $2 \times 10^8 \, \text{m/s}$.