POLARISATION

Polarisation of two interfering wave must be same state of polarisation or two source of light should be unpolarised.

BREWSTER LAW 
According to this law when unpolarised light is incident at polarising angle (i) on an interface separating a rarer medium from a denser medium, of refractive index μ as shown in Fig., below such that
    μ = tan i 
then light reflected in the rarer medium is completely polarised. Reflected and refractive rays are perpendicular to each other.

REDUCTION IN INTENSITY
Intensity of polarised light is 50% of that of the unpolarised light, i.e.,

     I_{p =} I_u / 2

where  I_p   = Intensity of polarised light and
             I_u   = Intensity of unpolarised light.