E X P E R I M E N T J
Keele University Physics/Astrophysics Laboratory
School of Physical and Geographical Sciences Experimental Scripts
72
Determination of the Brewster Angle
1. Introduction
Aim of this experiment is to determine the Brewster angle by measuring the degree of
polarisation as a function of angle of incidence. Light is partially polarised when reflected off a
dielectric surface and the degree of polarisation depends on the angle of incidence. At a particular
angle of incidence (the Brewster angle), the reflected light is completely plane polarised in the plane
parallel to the reflecting surface (Figure 1). The degree of polarisation of the laser beam reflected off
the flat side of an acrylic semi-circular („D‟) lens is measured by using an analyser and a light sensor. A
Rotary Motion Sensor mounted on the Spectrometer table is used to measure the angle of reflection
of the laser beam. A plot of the intensity of the reflected laser beam versus the angle of incidence can
be used to determine the Brewster angle. From this plot, determine the angle of incidence where the
intensity of the reflected laser beam reaches the minimum. This is the Brewster Angle, which is used
to calculate the refractive index of acrylic.
Theory
Light is partially polarised (Figure 1a) when reflected off a dielectric surface and the degree of
polarisation depends on the angle of incidence. At the particular angle of incidence (the Brewster
angle - θp ), the reflected light is completely plane polarised in the plane parallel to the reflecting
surface (Figure 1b). At the Brewster angle, the reflected beam is perpendicular to the refracted beam.
Using Snell's Law,
2211 sinsin
nn
(1)
where n1 and n2 are the refractive indices.
Figure 1: Ray diagrams for reflection and refraction at (a) θ1 < the Brewster angle (b) θ1 = the Brewster
angle (From Physics for Scientists and Engineers – Serway and Jewett)
