State what is meant by polarisation.
State why light waves can be plane polarised whereas sound waves cannot.
Two polarising filters A and B are arranged with their planes parallel to one another and perpendicular to a central axis line XY, as shown in Fig. 4.1. Filter A has a vertical transmission axis and filter B has a horizontal transmission axis. Unpolarised light of a single frequency travels along the line XY from a source at X. The light that leaves filter A is vertically plane polarised and has intensity $I_0$. Filter B is then turned about the line XY from its initial position. After this rotation, the intensity of the light leaving filter B is $\frac{1}{4} I_0$. Calculate the angle through which filter B is rotated from its initial position.
A microwave of intensity $I_0$ and amplitude $A_0$ meets another microwave of the same frequency and intensity $\frac{1}{4} I_0$, moving in the opposite direction. Both microwaves are vertically plane polarised and overlap where they meet. Explain, without calculation, why these two waves cannot produce a stationary wave with zero amplitude at its nodes.
Find, in terms of $A_0$, the maximum amplitude of the wave formed.