4.9PReflection of waves
Physics only — not on the Combined Science specification
Waves can be reflected at the boundary between two different materials. Waves can be absorbed or transmitted at the boundary between two different materials.
So three things can happen at a boundary, and usually all three happen at once in some proportion: some of the wave is reflected back, some is absorbed by the material, and some is transmitted through it.
You should be able to describe the effects of reflection, transmission and absorption of waves at material interfaces, and construct ray diagrams to illustrate the reflection of a wave at a surface.
For a ray diagram of reflection, the conventions are strict: draw the normal as a dashed line at right angles to the surface at the point where the ray hits, measure both angles from the normal rather than from the surface, and mark the direction of travel with an arrow on each ray. The angle of incidence equals the angle of reflection.
A smooth surface reflects all the rays in the same direction, giving a clear image — specular reflection. A rough surface reflects rays in many directions, because the normal at each point faces a different way, so no image forms — diffuse scattering.
Required practical 9
Investigate the reflection of light by different types of surface, and the refraction of light by different substances
- Reflection: place a plane mirror on paper, draw round it, and direct a ray from a ray box at the surface. Mark the incoming and reflected rays with two crosses each.
- Remove the mirror, join the crosses, draw the normal at the point of incidence, and measure the angles of incidence and reflection from the normal.
- Repeat for several angles of incidence, then repeat with a rough or matt surface in place of the mirror and describe what happens to the reflected light.
- Refraction: place a rectangular glass block on paper, draw round it, and direct a ray into one face. Mark the incoming ray and the ray leaving the far side.
- Remove the block, join the marks to show the path through the glass, draw the normal, and measure the angles of incidence and refraction.
- Repeat with blocks of different materials, such as glass and perspex, keeping the angle of incidence the same so the comparison is fair.
4.10–4.11What happens to a wave at a boundary
When a wave reaches the boundary between two materials, four things can happen to it, and usually several happen at once: it can be reflected, refracted, transmitted or absorbed. Which of them dominates depends on the two materials and on the wavelength of the wave.
Absorption transfers the wave's energy to the material's store of internal energy, so the material warms up. Transmission means the wave carries on through, though usually more slowly. Reflection sends it back into the material it came from. Refraction is transmission with a change of direction, and it is the one that needs explaining.
Refraction happens because the wave changes speed when it enters the new material, and the explanation has to be given in that order:
• the wave slows down (or speeds up) as it crosses into the new material • if it meets the boundary at an angle, one side of the wavefront reaches the boundary before the other and changes speed first • that makes the wavefront pivot, so the direction changes.
Which way it bends follows one rule. Going into a slower material, the wave bends TOWARDS the normal. Going into a faster material, it bends AWAY from the normal. A wave meeting the boundary along the normal — head on — still changes speed and wavelength, but does not change direction, because there is no side to reach the boundary first.
Different materials treat different wavelengths differently, and that is why the same boundary can behave in opposite ways for two waves. Glass transmits visible light and absorbs ultraviolet; a brick wall reflects visible light and transmits radio waves. When a question gives you two waves and one material, the answer is nearly always about wavelength.
Common mistake
Frequency does not change at a boundary. It is fixed by whatever produced the wave, so when the speed changes it is the wavelength that must change to match — writing that the frequency drops in glass is a guaranteed lost mark.
Before you practise
- Four things can happen at a boundary: reflection, refraction, transmission and absorption — usually several at once.
- The angle of incidence equals the angle of reflection, and both are measured from the normal.
- The normal is a dashed line at 90° to the surface at the point where the ray hits it.
- Specular reflection is from a smooth surface and gives a clear image; diffuse reflection from a rough one scatters the light.
- Refraction happens because the wave changes speed on crossing the boundary.
- Into a slower material a wave bends towards the normal; into a faster one, away from it.
- Frequency never changes at a boundary — speed and wavelength change together.