Section 1 of 6What can happen at a boundary
EM wave meets a new material
Reflection
- Reflection The wave bounces off the surface. Shiny, smooth surfaces reflect most visible light.
Absorption
- Absorption The wave's energy is taken in by the material, which usually warms up.
Transmission
- Transmission The wave passes through the material. Glass transmits visible light.
Refraction
- Refraction The wave changes direction and speed as it enters a material with a different density, unless it enters along the normal.
Section 2 of 6Same wave, different materials
| Material | Visible light | Radio waves | X-rays |
|---|---|---|---|
| Glass | Transmitted | Mostly transmitted | Mostly transmitted |
| Metal | Reflected | Reflected or absorbed, so signals are blocked | Absorbed by thick metal such as lead |
| Bone | Absorbed or reflected | Mostly transmitted | Absorbed |
| Soft tissue | Partly absorbed | Transmitted | Mostly transmitted |
Section 3 of 6Why refraction occurs (HT only)
1Speed changes
- Speed changes The speed of a wave depends on the material. Light is slower in glass than in air.
2Frequency fixed
- Frequency fixed The frequency is set by the source and does not change when the wave enters a new medium.
3Wavelength changes
- Wavelength changes Since v = f × λ and v changes while f is fixed, the wavelength changes with the speed.
4Direction bends
- Direction bends If the wave enters at an angle, one side of the wavefront slows first, so the wave turns towards the normal in the slower medium.
Section 4 of 6Reflection and absorption in practice
Mirrors
- Mirrors A smooth surface gives specular reflection, where parallel rays stay parallel.
Rough surfaces
- Rough surfaces A rough surface scatters light in many directions, called diffuse reflection.
Atmosphere
- Atmosphere Ozone absorbs most UV;
- the atmosphere is transparent to visible light and some radio waves, but absorbs a lot of infrared.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Transmission | A wave passing through a material. |
| Absorption | A wave transferring its energy to the material. |
| Reflection | A wave bouncing off a surface. |
| Refraction | A change in direction when a wave passes from one medium into another with a different speed. |
| Normal | An imaginary line at right angles to a surface at the point where a ray meets it. |
Section 6 of 6Exam technique
1
Saying that frequency changes during refraction. Only speed and wavelength change.
2
Drawing the refracted ray bending away from the normal when light slows down. It bends towards the normal.
3
Assuming that a material that is transparent to light is transparent to all EM waves. It depends on the wavelength.
4
Mixing up absorption and reflection; an absorbing surface looks dark and warms up.