7.2 Transverse Waves: EM Spectrum & Polarisation

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.

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.