Polarisation (CIE A Level Physics)

A beam of vertically polarised monochromatic light is incident on a polarising filter, as shown in Fig. 5.1.

Fig. 5.1

The transmission axis of the filter is initially vertical and the transmitted light beam has the same intensity as the incident light beam.

The filter may be rotated about the direction of the light beam to change the angle of the transmission axis against the vertical.

State one angle of the transmission axis to the vertical that results in no transmitted light beam.

angle = ................................... ° 

The filter is now positioned with its transmission axis at angle θ to the vertical, as shown in Fig. 5.2.

Fig. 5.2

The ratio    is equal to 0.75.

θ = ...................................... ° [2]

.

ratio = ..................................... [2]

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.

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.

A transverse wave passes straight through a polariser. A and B are two points on the wave a distance of λ apart. The displacement of the wave at the point when A passes through the polariser is at its maximum.

Sketch a possible graph to show the displacement of the wave against phase as it passes through the polariser, θ and label points A and B. 

Electromagnetic waves are transverse waves that carry energy through the vacuum of space as progressive transverse waves. 

Discuss the motion of an electromagnetic wave with reference to the appropriate fields.

Plane-polarised electromagnetic waves are incident on an aerial positioned to give a maximum response. High winds cause the aerial to rotate anti-clockwise about the plane of the incident wave until it makes an angle of 40° out of the plane of wave oscillation.

Calculate the percentage reduction in the amplitude of the signal received by the aerial following the high winds.

The intensity of incident radiation is 0.06 mW m⁻². When the aerial is repaired, it is found to have a vibration amplitude of 5.4 mm.

Determine the angle at which the aerial has been set.

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.