CAIE A-Level Physics 7.5 Polarisation

Practise explaining polarisation as evidence for transverse waves and calculating transmitted intensity through single or multiple filters with Malus’s law.

Exam points

explain polarisation as evidence of transverse-wave behaviour and distinguish it from longitudinal behaviour
calculate and interpret transmitted intensity through polarising filters with Malus’s law, including multiple filters

Covered syllabus points

7.5.1—Polarisation is a phenomenon associated with transverse waves 7.5.2—Malus’s law (I = I0 cos2θ ) to calculate the intensity of a plane-polarised

[Maximum number: 5]

(a)

Polarisation is a phenomenon associated with light waves but not with sound waves.

[ 2 ]

(i)

State the meaning of polarisation.

[ 1 ]

(ii)

State why light waves can be plane polarised but sound waves cannot.

[ 1 ]

(b)

Two polarising filters A and B are positioned so that their planes are parallel to each other and perpendicular to a central axis line XY , as shown in Fig. 4.1.

Fig. 4.1

The transmission axis of filter A is vertical and the transmission axis of filter B is horizontal.

Unpolarised light of a single frequency is directed along the line XY from a source positioned at X . The light emerging from filter A is vertically plane polarised and has intensity $I_{0}$ .

Filter B is rotated from its starting position about the line XY , as shown in Fig. 4.1. After rotation, the intensity of the light emerging from filter B is $\frac{1}{4} I_{0}$ .

Calculate the angle of rotation of filter B from its starting position.
angle of rotation = ${ }^{\circ}$

[ 3 ]

$I=I_{0} \cos ^{2} \theta$

C1

$\cos ^{2} \theta=1 / 4 \text { so } \cos \theta=1 / 2$ $\theta=60^{\circ}$ or $120^{\circ}$ or $240^{\circ}$ or $300^{\circ}$

C1

angle of rotation $=\left(120^{\circ}-90^{\circ}\right)$ or $\left(240^{\circ}-90^{\circ}\right)$ or $\left(300^{\circ}-90^{\circ}\right)$ $=30^{\circ} \text { or } 150^{\circ} \text { or } 210^{\circ} \text { or } 330^{\circ}$

A1

[Maximum number: 4]

(a)

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

Fig. 5.1

[ 4 ]

(i)

The transmission axis of the filter is initially vertical. The filter is then rotated through an angle of $360^{\circ}$ while the plane of the filter remains perpendicular to the beam.

On Fig. 5.2, sketch a graph to show the variation of the intensity of the light in the transmitted beam with the angle through which the transmission axis is rotated.

Fig. 5.2

[ 2 ]

light intensity has maximum value at $0^{\circ}, 180^{\circ}, 360^{\circ}$ and zero intensity at $90^{\circ}, 270^{\circ}$

M1

'sinusoidally-shaped' curve

A1

(ii)

The intensity of the light in the incident beam is $7.6 \mathrm{Wm}^{-2}$ . When the transmission axis of the filter is at angle $\theta$ to the vertical, the light intensity of the transmitted beam is $4.2 \mathrm{Wm}^{-2}$ .

Calculate angle $\theta$ .

[ 2 ]

$4.2=7.6 \cos ^{2} \theta$

C1

$\theta=42^{\circ}$

A1

All question bank results loaded

Covered syllabus points

7.5 Polarisation question 1

Question (a)

Question (i)

Question (ii)

Question (b)

7.5 Polarisation question 2

Question (a)

Question (i)

Question (ii)