# 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?

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'.

• Observation: What are the conditions that we can make red light disappear in a real situation, in a real experiment like this? In other words, the question is good, but the picture awful. Just making more questions than answers. But hey, let's have a question like this: You have two laser pointers of the same colour, a glass plate and a mirror. How can you make a laser beam disappear? That is an experiment that can be constructed, theoretically solved and perhaps even successfully performed.*

5. What is the wavelength of green light?

500 nm

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

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