An unpolarised light beam strikes a glass surface at Brewster's angle. Then

The correct option is: The reflected light will be completely polarised but the refracted light will be partially polarised.

To understand why this happens, let us break down the physical phenomenon of light reflection and refraction at Brewster's angle step-by-step:

1. Nature of Unpolarised Light:
An unpolarised light beam consists of electric field oscillations occurring in all possible directions perpendicular to the direction of propagation. We can resolve these oscillations into two orthogonal components:
• One component vibrating parallel to the plane of incidence (known as p-polarised light).
• Another component vibrating perpendicular to the plane of incidence (known as s-polarised light, parallel to the surface).

2. Brewster's Law and Brewster's Angle:
When light strikes a boundary between two media with different refractive indices, a portion is reflected and a portion is refracted (transmitted).
Brewster's angle (also known as the polarisation angle, ${\theta }_p$) is a specific angle of incidence at which light with a particular polarisation (parallel to the plane of incidence) is perfectly transmitted through the transparent surface without any reflection.

3. Mechanism at Brewster's Angle:
At Brewster's angle, the angle between the reflected ray and the refracted ray is exactly 90 degrees:
${\theta }_p+{\theta }_r=90°$
where ${\theta }_r$ is the angle of refraction.
The electric field vibrations of the refracted light drive the electrons in the medium, which then oscillate and radiate light to produce the reflected wave. Because these dipoles oscillate parallel to the direction of propagation of the reflected wave, they cannot radiate energy in that direction. As a result, the component of light vibrating parallel to the plane of incidence (p-polarised) is not reflected at all.

4. State of Polarisation of the Rays:
• Reflected Light: Since the parallel component cannot reflect at this specific angle, only the perpendicular component (s-polarised) is reflected. Therefore, the reflected beam contains vibrations in only one direction (perpendicular to the plane of incidence), making the reflected light completely polarised.
• Refracted Light: The refracted beam contains all of the parallel component (since none was reflected) and the remaining portion of the perpendicular component that did not reflect. Because it still contains electric field oscillations in both orthogonal directions (though unequal in magnitude), the refracted light is partially polarised.