Brewster's angle:
Ordinary light, when allowed to undergo refraction, the partially reflected light gets partially plane polarized. However, there is an angle of incidence, at which an ordinary light undergoes refraction as well as reflection (partial) and then the partially reflected ray is richly plane polarized. Such an angle is known as polarizing angle or Brewster's angle. It is denoted by i_p.
Light can be polarized by reflecting it from a transparent medium. The extent of polarization depends on the angle of incident. At a particular angle of incidence, called Brewster's angle, the reflected light is completely polarized as shown in the diagram below:

Polaroid is a thin and large sheet made of crystalline polarizing material which is used to produces plane polarized beam of light. 

(i) Polaroids can be used to demonstrate the transverse nature of light waves.

• The electric field, associated with propagation of light waves is always perpendicular to the direction of propagation of the wave.
• If a light wave is incident on the polaroid, the electric vectors assosciated with the propagating wave, along the direction of the aligned molecules get absorbed. 

(ii) If an unpolarized light wave is incident on a polaroid then, the the light wave will get linearly polarised with the electric vector oscillating along a direction perpendicular to the aligned molecules.

The fringe width will be very small (almost negligible) and fringes will not be separately visible if the separation between the two coherent sources is large.

Given, 
Fringe width, $\mathrm{\beta }$ = 2 mm 
Wavelength of light used, $\mathrm{\lambda }$ = 6000 $\stackrel{\mathrm{o}}{\mathrm{A}}$ 

Formula for fringe width in air,   $\mathrm{\beta } = \frac{\mathrm{D\lambda }}{\mathrm{d}}$ 

Fringe width in liquid is, $\mathrm{\beta }\text{'} = \frac{D\lambda \text{'}}{\mathrm{d}} = \frac{D\lambda }{d\mu }$

Refractive index of the medium, $\mathrm{\mu } = 1.33$ (given)

Therefore,
                            $\mathrm{\beta }\text{'} = \frac{\mathrm{\beta }}{\mathrm{\mu }}$ 

i.e.,                      $\mathrm{\beta }\text{'} = \frac{2.0}{1.33} = 1.5 \mathrm{mm}.$