P1bL2 Waves and Matter
Key Words
Absorb - soak up
Frequency - number of waves per second
Gamma rays - very energetic (very short wave) electromagnetic radiation from nuclei.
Infra red radiation - radiation given out by warm objects with slightly longer wavelength than visible red light.
Microwaves - short wavelength radio waves, wave length of about 1 - 10 cm.
Opaque - blocks off waves; they cannot pass through the material.
Radio waves - long wavelength electromagnetic radiation given out by high frequency alternating currents.
Reflected - waves bouncing off a shiny surface.
Transparent - waves can pass through the material.
Rays - beams of radiation.
Spectrum - a family of waves where one kind of wave runs into another without a clear boundary.
Ultra-violet - short wavelength light, with wavelength slightly shorter than visible violet light.
Visible light - light of a wavelength our eyes can detect.
Wavelength - length of between two successive crests.
Wave speed - how fast a wave travels at.
X-rays - short wavelength radiation less energetic than gamma rays, but more energetic than ultraviolet.
Grade E
Electromagnetic waves can bounce off objects. They are reflected. A mirror reflects light, and the angle of incidence = angle of reflection.
Electromagnetic waves can be absorbed by materials:
- You absorb infra red from a fire and you feel warm;
- Food in a microwave oven warms up because the water molecules absorb the radiation and vibrate more.
- Dark surfaces are good at absorbing EM waves, especially light.
Some radiations, especially with short wavelengths (and high energy) pass through objects. Their energy is not transferred to the particles that make up the objects.
Grade C
How EM waves interact with materials depends on the energy of the wave and the type of material. For example, bricks are transparent to radio waves, while they are opaque to light waves. That is why you can hear the radio in your house, but you cannot see what the neighbours are up to.
| Wave | Behaviour with materials |
| Radio waves |
Radio waves can pass through the atmosphere, but longer wave radio waves are reflected by the ionosphere. They pass through walls and our bodies. |
| Microwaves | These pass through the atmosphere and the ionosphere. Water molecules absorb them and gain energy. In early experiments with powerful radars, birds flying across the beams dropped out of the sky partially cooked. |
| Infra red | Absorbed to a limited depth by human skin. |
| Light | Strongly absorbed by our bodies, but any heating effect is very slight. |
| Ultra violet | UV light is absorbed. Some ionisation can occur, leading to damage to cells (sunburn) and DNA. |
| X-rays | Soft tissues are transparent to X-rays, but they are absorbed by bones. Ionisation can occur, leading to tissue damage. |
| Gamma rays | Pass through virtually anything. They can ionise atoms in our tissues, leading to damage. |
Grade A
The wave equation links wave speed with the wavelength and the frequency:
wave speed (m/s) = frequency (Hz) × wavelength (m)
In Physics code:
c = fl
In triangle form:
The strange looking letter, l, that looks like an upside-down y is lambda, a Greek letter 'l'.
Some wave speeds are shown in the table below:
| Wave | Speed (m/s) |
| EM waves | 300 000 000 |
| Sound in air | 340 |
| Sound in water | 1500 |
| Sound in steel | 5000 |