A-level Physics (Advancing Physics)/What is a wave?/Worked Solutions

1. Through what medium are sound waves propagated?

Usually the air, but most material objects will carry sound.

2. What aspects of the behaviour of light make it look like a wave?

Superposition and diffraction (and others).

3. What aspects of the behaviour of light make it look like a particle?

Photoelectric effect

4. Consider the diagram on the right. White light is partially reflected by the transparent material. Some of the light, however, is refracted into the transparent material and reflected back by the opaque material. The result is two waves travelling in the same place at the same time at the same polarisation(the light is not a single beam). Why does, say, the red light disappear?

If the refractive index and width of the transparent material are correct, then the two waveforms leaving it will be half a wavelength apart, as one will have travelled further. Destructive interference will cause the red light to become 'invisible'.

5. What is the wavelength of green light?

500nm

6. The lowest frequency sound wave humans can hear has a frequency of approximately 20Hz. Given that the speed of sound in air is 343ms^-1, what is the wavelength of the lowest frequency human-audible sound?

$\lambda ={\frac {v}{f}}={\frac {343}{20}}=17.15{\mbox{m}}$