9702/21/O/N/22: (a) Polarisation is a phenomenon associated with light waves but not with sound waves.

(i) State the meaning of polarisation.
(ii) State why light waves can be plane polarised but sound waves cannot.

(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. 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 I0.

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 1
4 I0. Calculate the angle of rotation of filter B from its starting position.

(c) A microwave of intensity I0 and amplitude A0 meets another microwave of the same frequency and of intensity 1/4 I0 travelling in the opposite direction. Both microwaves are vertically plane
polarised and superpose where they meet.

(i) Explain, without calculation, why these two waves cannot form a stationary wave with zero amplitude at its nodes.

(ii) Determine, in terms of A0, the maximum amplitude of the wave formed.

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