Four ways a substance affects a wave

1. When an electromagnetic wave reaches the surface of a substance, it can be reflected, transmitted, refracted or absorbed.
2. Reflection sends the wave back from the interface into the material it came from, as happens when light meets a mirror.
3. Transmission lets the wave carry on through the substance and out of the other side, as light does through a clean window.
4. Refraction is a change in the direction of a transmitted wave as it crosses an interface at an angle.
5. Absorption transfers the energy carried by the wave to the substance, which usually increases its thermal energy store so that it warms up.
6. These four effects are not alternatives where only one can happen, because a single beam is normally shared between several of them at the same interface.
7. Light striking a window pane is mostly transmitted, but a small fraction is reflected from each surface of the glass and a little is absorbed inside it.
8. The reflected fraction is the reason you can see a faint reflection of yourself in a shop window while most of the light still passes straight through.
9. Energy is conserved at the interface, so the energy reflected, transmitted and absorbed always adds up to the energy carried by the incident wave.

Why wavelength matters

1. Different parts of the electromagnetic spectrum have very different wavelengths, from radio waves that can be kilometres long to gamma rays smaller than an atom.
2. The way a substance responds to a wave depends on the wavelength of the wave as well as on the substance itself.
3. One substance can therefore transmit one wavelength, absorb a second wavelength and reflect a third.
4. Words such as transparent and opaque only make sense when you also say which wavelength you are describing.
5. Ordinary window glass is transparent to visible light, but it absorbs most ultraviolet and much of the infrared at longer wavelengths.
6. This is why you are unlikely to get sunburnt sitting behind a closed window, even on a bright summer day.
7. It also explains why a greenhouse or a parked car warms up in the sun, because visible light passes in through the glass and is absorbed by the plants or the seats.
8. The warmed surfaces then give out longer-wavelength infrared, which the glass absorbs rather than transmits, so the energy stays inside.
9. A brick wall is opaque to visible light, but it transmits most radio waves, which is why a radio still picks up a station inside a house.
10. Metal surfaces reflect radio waves and microwaves strongly, so the metal mesh in the door of a microwave oven keeps the microwaves inside.
11. The holes in that mesh are much smaller than the wavelength of the microwaves but much larger than the wavelength of visible light, so you can still see your food cooking.
12. Water molecules absorb microwaves of the wavelength used in a microwave oven, so the water in food gains energy and the food heats up.
13. Soft tissue in the body transmits most X-rays, while bone absorbs them much more strongly, which is the idea behind an X-ray photograph.
14. The Earth's atmosphere transmits visible light and most radio waves, but the ozone layer absorbs much of the Sun's ultraviolet and gases such as carbon dioxide absorb some infrared.

Refraction varies with wavelength too

1. Refraction also depends on wavelength, so different wavelengths change direction by slightly different amounts when they cross the same interface.
2. When a narrow beam of white light passes through a glass prism, violet light is refracted through a larger angle than red light.
3. The colours that make up white light therefore separate and fan out into a spectrum on a screen placed beyond the prism.
4. Water droplets in the air separate sunlight in the same way, which is how the colours of a rainbow appear.