Waves

Polarisation of Waves

When a transverse wave travels through a medium, the plane in which the displacement of the medium occurs is called the plane of polarisation of the wave. For example, if you shake the end of a stretched spring up and down vertically, you will produce vertically polarised waves on the spring.

It should be clear that the concept of polarisation only has meaning for transverse waves.

Electro-magnetic radiations, like light, radio waves etc are considered to consist of linked electric and magnetic fields at 90° to each other and at 90° to the direction of propagation of the wave, as shown below.

By convention the plane of polarisation of an E-M wave is the plane of the electric field.

Normal light sources produce a “mixture” of waves with their electric fields in all planes perpendicular to the direction of propagation. They are said to produce unpolarised (or randomly polarised) light.

If the electric field is in one plane only, then the light is said to be (plane) polarised.

The process of selecting waves having their “oscillations” in a given plane is called polarising the waves.

Polarisation by Reflection

When light is reflected at the surface of a piece of transparent material, it is found to be partially (plane) polarised: a greater proportion of the reflected light has its electric field parallel to the reflecting surface than in the incident beam.

When the angle of incidence is such that the reflected light and the refracted light are at 90° to each other, the reflected light is totally (plane) polarised: it only contains waves with their electric field parallel to the reflecting surface. (The refracted light is also partially (plane) polarised as it no longer contains a totally random “mix” of polarisations.)

This angle of incidence is called the polarising angle*, i_p.

Snell’s law states that

where n is the refractive index corresponding to the two media

but in this case angle
so
which means

*the polarising angle is also called the Brewster angle

• Waves Chapters Index
• Malus’ Law
• Uses of Polarised light