Question

(a) Define wavefront. Use Huygens' principle to verify the laws of refraction.
(b) How is linearly polarised light obtained by the process of scattering of light ? Find the Brewster angle for air-glass interface, when the refractive index of glass =1.5.

Answer 1

(a) Wavefront : A wavefront is a continuous locus of all the particles of a medium which are vibrating in the same phase.
In $\Delta A{A}^{\prime }B$,
$sini=\frac{A^{\prime }B}{AB}...\left(i\right)$
In $\Delta A{B}^{\prime }B$,
$sinr=\frac{A{B}^{\prime }}{AB}...\left(ii\right)$
Dividing equation (i) by (ii),
$\frac{sini}{sinr}=\frac{A^{\prime }B/AB}{A{B}^{\prime }/AB}\frac{sini}{sinr}=\frac{A^{\prime }B}{A{B}^{\prime }}=\frac{ct}{vt}\frac{sini}{sinr}=\frac{c}{v}=\mu$
This is the law of refraction.
(b) When a beam of white light is passed through a medium containing particles whose size is of the order of wavelength of light, then the beam gets scattered. When the scattered light is viewed through an analyser in a direction at the right angle to the direction of incidence, it is found to be plane polarised. This is called polarisation by scattering.
We know that, $tan{i}_p=\mu \Rightarrow tan{i}_p=1.5\Rightarrow i_p=ta{n}^{-1}\left(1.5\right)={56.31}^{\circ }$