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.

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.

Draw the shape of a string when it is vibrating in its second harmonic mode.

A string held under tension and has a first harmonic frequency of $250 \mathrm{Hz}$. The tension in the string is changed so that the speed increases by $\sqrt{2}$. Predict the new frequency of the first harmonic.