Polarisation (CIE A Level Physics)

When electromagnetic waves are reflected from a shiny surface, such as a road sign, they often become polarised. 

Suggest how to determine experimentally if visible light reflected from a road sign is polarised.

Three polaroid filters P₁, P₂ and P₃ are aligned as shown in Fig. 1.1. 

Unpolarised light is incident on P₁ and subsequently passes through each of the three polaroid filters. P₁ and P₂ are fixed, with P₂ at 20° to P₁, but P₃ can be rotated to any angle θ  to that of P₁.

Fig. 1.1

Determine the angles of θ at which minima and maxima of emergent light intensity occur.

Table 1.1 lists different waves, whether they are polarised or unpolarised and a reason. 

State, by completing the spaces in the table whether the waves listed are polarised or unpolarised, and give a reason for your answer. 

Table 1.1

Electromagnetic waves, being transverse, can also be polarised.

A light source is viewed through two pieces of polarisers, P and Q, with their axes initially at radians from each other, as shown in Fig. 1.1.

Fig. 1.1

Using the axes below, sketch the variation of the intensity of light reaching the eye with angular displacement of Q with respect to P when polariser Q is rotated. 

Unpolarised light is passed through a pair of polarising sunglasses as shown in Fig. 1.1.

The light becomes plane polarised.

Explain the difference between unpolarised and plane polarised light.

In an experiment to investigate polarising filters, two square filters are used.

Initially, they are placed between the source of unpolarised light and a hand-held digital light meter, as shown in Fig. 1.2. The planes of polarisation of the filters are indicated by double-headed arrows.

The light meter measures the intensity of radiation as 1.00 W m⁻².

One of the polarising filters is then rotated, as shown.

Sketch a graph using the axes as shown in Fig. 1.3. to show how the intensity of light measured by the light meter varies as the filter is rotated from 0° to 90°.

Polarisation can be used to identify when some materials are under stress. Placing the material under stress causes the plane of polarisation of the light passing through the material to be rotated. The greater the stress, the greater the rotation of the plane of polarisation of the light.

A piece of transparent material under stress is positioned between two polarising filters. There is an angle of 90^o between the planes of polarisation of the two polarising filters.

Explain how this photograph can be used to identify areas of the material with different amounts of stress.

Fig. 1.1. shows two ways in which a wave can be made to travel along a slinky spring.

Use arrows to show how points Q and R will move next as the two waves move to the right.

Use the diagram in Fig. 1.1.  

Light from a filament lamp is viewed through two polarising filters A and B as shown in Fig. 1.2.

The direction of plane polarisation is indicated on each filter using a double-headed arrow.

Explain why the observer cannot see light from the filament lamp.

Polaroid sunglasses are sold as sporting equipment to people who fish, sail and kayak.

Explain how polaroid sunglasses can be used to more clearly see objects such as fish, rocks and other obstacles which are under the surface of water, by a person who is above the water.