# When defined as an angle at which an incident

When light is incident on a transparent solid material, one
part of it gets reflected and another part gets refracted. If light is incident
on the interface between the two media such that there is 90° angle between the
reflected and refracted rays, the reflected light will be linearly polarized.

Brewster’s angle, or the
polarizing angle, is defined as an angle at which an incident beam of
unpolarized light is reflected after complete polarization.1

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Background

In his studies on polarized light, Brewster discovered that when light
strikes a reflective surface at a certain angle, the light reflected from that
surface is plane-polarized. He elucidated a simple relationship between the
incident angle of the light beam and the refractive index of the reflecting
material. When the angle between the incident beam and the refracted beam
equals 90 degrees, the reflected light becomes polarized. This rule is often
used to determine the refractive index of materials that are opaque or
available only in small quantities.2

Brewster’s Angle and Polarized Light

When considering the incidence of non-polarized light on a flat
insulating surface, there is a unique angle at which the reflected light waves
are all polarized into a single plane. This angle is commonly referred to
as Brewster’s angle, and can be easily calculated utilizing the
following equation for a beam of light traveling through air:

n = sin (?i)/sin
(?r) = sin (?i)/sin (?90-i) = tan (?i)

Where n is the refractive index of the medium from
which the light is reflected, ?i is the angle of
incidence, and ?r is the angle of refraction. By
examining the equation, it becomes obvious that the refractive index of an
unknown specimen can be determined by the Brewster angle. This feature is
particularly useful in the case of opaque materials that have high absorption
coefficients for transmitted light, rendering the usual Snell’s law formula
inapplicable. Determining the amount of polarization
through reflection techniques also eases the search for the polarizing axis of
a sheet of polarizing film that is not marked.

For water (refractive index of 1.333) and glass (refractive index of
1.515 the critical (Brewster’s) angles are 53 and 57, degrees, respectively.

Hypothesis/Theory:

“When a beam of unpolarized light reflects
from a surface at the Brewster angle, the reflected beam will be polarized
along a direction parallel to the surface. The Brewster angle is the angle of
incidence that results in a 90° angle between the reflected and refracted
beams.”3

It is hypothesised that as the refractive index changes it will affect
Brewster’s angle.

Design and Methodology:

Equipment;

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