6.2.2a Refraction and wavelength-dependent behaviour of EM waves (HT)

The same material treats different waves differently

1. A substance can absorb, transmit, reflect or refract an electromagnetic wave, and which of these happens can depend on the wave’s wavelength.
2. So the same material may transmit one range of wavelengths while it absorbs or reflects another range.
3. This is why a material should never be called simply “transparent” or “opaque” without saying which wavelength you mean: glass transmits visible light but absorbs much of the ultraviolet in sunlight.

Refraction happens because the wave changes speed

1. An electromagnetic wave can travel at different speeds in different substances, so its speed may change as it crosses a boundary.
2. The normal is an imaginary line drawn at $90^{\circ }$ to the boundary at the point where the wave meets it.
3. If the wave slows down, it bends towards the normal.
4. If the wave speeds up, it bends away from the normal.
5. If the wave meets the boundary along the normal, its speed still changes but its direction does not.
6. The frequency of the wave does not change at a boundary.
7. Because $v=f\lambda$ and the frequency $f$ is fixed, a change in speed $v$ must be matched by a change in wavelength $\lambda$.
8. So when the wave slows down its wavelength decreases, and when it speeds up its wavelength increases.

Explaining refraction with wave fronts

1. Wave fronts are drawn as a set of parallel lines; the spacing between them is the wavelength, and the wave travels at $90^{\circ }$ to them.
2. When a wave front reaches a slower medium at an angle, the end that crosses the boundary first slows down first.
3. The rest of that wave front is still in the faster medium, so it keeps moving faster for a moment.
4. This makes the wave front pivot, which turns the direction of travel towards the normal.
5. Because the frequency is unchanged, the slower speed also pulls the wave fronts closer together, showing the shorter wavelength.

6.2.2b Refraction ray diagrams and infrared radiation (required practical)

Drawing refraction: bend at the boundary, measure from the normal

1. When light passes from air into glass at an angle, it changes speed and so changes direction.
2. If the wave slows down, it bends towards the normal.
3. If the wave speeds up, it bends away from the normal.
4. If the wave travels along the normal, its speed changes but its direction does not.
5. The normal is a line drawn at $90^{\circ }$ to the boundary at the point where the ray meets it.

Constructing a refraction ray diagram

1. Draw the boundary between the two media, and mark the point where the ray meets it.
2. Draw a dashed normal at $90^{\circ }$ to the boundary at that point.
3. Draw the incident ray approaching the boundary, with an arrow showing its direction.
4. Draw the refracted ray leaving the boundary, also with a direction arrow.
5. If the wave slows down, draw the refracted ray closer to the normal; if it speeds up, draw it further from the normal.
6. Measure the angle of incidence and angle of refraction from the normal, never from the boundary.

Infrared and surfaces: dark and matt wins both ways

1. All objects absorb and emit infrared radiation, and the amount depends on the nature of the surface.
2. Dark, matt surfaces are the best absorbers and the best emitters of infrared radiation.
3. Light, shiny surfaces are poor absorbers and poor emitters, and are good reflectors of infrared radiation.

1. A good absorber is always a good emitter, so a matt black surface both soaks up and radiates infrared most readily.