The speed of sound in air is $340 \mathrm{m} {\mathrm{s}}^{-1}$ and in water it is $1500 \mathrm{m} {\mathrm{s}}^{-1}$. Determine the angle at which a beam of sound waves must hit the air–water boundary so that no sound is transmitted into the water.

A radio station, $R$, emits radio waves of wavelength $1600 \mathrm{m}$ which reach a house, $H$, directly and after reflecting from a mountain, $M$, behind the house (see diagram). The reception at the house is very poor. Estimate the shortest possible distance between the house and the mountain. (Pay attention to phase changes.)

Describe what is meant by a standing wave. List the ways in which a standing wave differs from a travelling wave.

Outline how a standing wave is formed.

In the context of standing waves describe what is meant by node.

In the context of standing waves describe what is meant by antinode.

In the context of standing waves describe what is meant by wave speed.

Describe how you would arrange for a string that is kept under tension, with both ends fixed, to vibrate in its second harmonic mode.