Properties and Hazards of Electromagnetic Waves

When an electromagnetic (EM) wave travelling through one material reaches the surface of a different material, four things can happen to it. The wave can be absorbed (its energy is transferred to the material), transmitted (it passes through), refracted (it changes direction as it passes through), or reflected (it bounces back).

(Higher Tier) How much of each effect occurs depends on the wavelength of the wave and on the material it meets. The same surface can treat different EM waves completely differently.

Glass is the clearest example. Visible light is mostly transmitted through window glass, which is why you can see through it. Ultraviolet with a shorter wavelength is largely absorbed by the same glass, which is why you do not get sunburnt sitting behind a closed window. Longer-wavelength infrared behaves differently again. One property changes, wavelength, and the outcome changes with it.

Effect at a boundary	What happens to the wave
Absorbed	Energy is transferred to the material
Transmitted	The wave passes through the material
Refracted	The wave changes direction on entering the material
Reflected	The wave bounces back off the surface

Refraction is the change in direction of a wave as it crosses a boundary between two materials. It is caused by one thing: the wave changes speed when it moves into a different substance.

(Higher Tier) Some effects at a boundary, such as refraction, happen because the wave travels at different speeds in different substances.

When a wave slows down on entering a denser material, and it meets the boundary at an angle, it bends towards the normal (the line drawn at 90° to the surface). When it speeds up on leaving into a less dense material, it bends away from the normal. A wave that hits the boundary along the normal (head-on, at 0°) changes speed but does not change direction, so it does not bend.

The frequency of the wave stays the same during refraction. Because wave speed $v=f\lambda$, if the speed changes but the frequency does not, the wavelength must change to match.

You must recall and apply $v=f\lambda$ (wave speed = frequency × wavelength).

A ray diagram shows this clearly. The incident ray meets the surface, and the refracted ray inside the denser material lies closer to the normal.

        normal
          |
   incident ray
       \  |
        \ |
─────────\|─────────  boundary (air → glass)
          |\
          | \  refracted ray
          |  \  (bent towards the normal)

(Higher Tier only) You may be asked to use a wave-front diagram to explain refraction as a change of speed.

A wave front is a line joining points on a wave that are all at the same stage of their cycle, for example the crests. Wave fronts are drawn at right angles to the direction the wave travels, and the gap between them is one wavelength.

Picture parallel wave fronts approaching a boundary at an angle, moving from a fast material into a slower one. The end of each wave front that crosses into the slower material first begins to travel more slowly, while the rest of the wave front is still moving quickly in the fast material. That side of the front lags behind. Because one end slows before the other, the whole set of wave fronts pivots, and the direction of travel swings towards the normal.

This is the mechanism behind refraction: unequal speeds along a wave front make it turn. The wave fronts also sit closer together in the slower material, because the wavelength shortens while the frequency stays fixed.

The direction of travel always stays at 90° to the wave fronts, so bending the fronts bends the ray.

Different parts of the EM spectrum are produced in different ways, and the exam asks specifically about two of them.

(Higher Tier) Radio waves can be produced by oscillations in electrical circuits. When an alternating current flows in a circuit, the moving charges create a radio wave with the same frequency as the oscillations.

The process also works in reverse. When radio waves are absorbed by a conductor, such as an aerial, they may induce an alternating current in it with the same frequency as the radio wave itself. This is how a radio or television aerial picks up a broadcast signal.

At the other end of the spectrum, the origin is nuclear, not electrical. Changes in atoms and the nuclei of atoms can generate and absorb EM radiation over a very wide range of frequencies. In particular:

Gamma rays are produced by changes in the nucleus of an atom. They come from the nucleus, not from electron movements or from circuits.

This nuclear origin is why gamma rays are linked to radioactive decay, and it is a common one-mark recall question.

The high-frequency, short-wavelength end of the spectrum carries the most energy per wave, and this is what makes it hazardous to living tissue.

The effect of any radiation on the body depends on the dose, and radiation dose is measured in sieverts (Sv). A larger dose means a greater risk of harm. (You do not need to recall the definition of the sievert, only its name and that dose measures risk.)

Ultraviolet, X-rays and gamma rays can all be harmful to human body tissue:

Radiation	Main hazard
Ultraviolet	Can age the skin prematurely and increase the risk of skin cancer
X-rays	Ionising; can cause gene mutation and cancer
Gamma rays	Ionising; can cause gene mutation and cancer

Ionising means the radiation carries enough energy to knock electrons off atoms, turning them into charged ions. Damaging the atoms inside cells in this way can alter the DNA, causing gene mutation that may lead to cancer. When a question gives you data on exposure and risk, read the figures directly from the table or graph and draw a conclusion supported by those numbers, rather than relying on general statements.

This required practical investigates how the infrared radiation absorbed or radiated by a surface depends on the nature of that surface.

A common version uses a Leslie cube: a hollow metal cube with four differently finished faces, typically matt black, shiny black, matt white and shiny silver. The cube is filled with hot water so every face is at the same temperature.

Method (emission):

1. Fill the Leslie cube with boiling water and let the temperature settle.
2. Hold an infrared detector (an infrared thermometer or a thermopile connected to a meter) the same distance from each face in turn.
3. Record the detector reading for each of the four surfaces.

Variables:

Variable	In this experiment
Independent	The type of surface (matt black, shiny silver, and so on)
Dependent	The infrared radiation detected from the surface
Control	Temperature of the water, distance from face to detector, same detector

Expected result: the matt black surface emits the most infrared radiation and the shiny silver surface emits the least. A parallel investigation of absorption (shining a heater on differently coloured surfaces with a thermometer behind each) shows matt black surfaces also absorb the most.

Each face must be the same distance from the detector and the water the same temperature; otherwise the surface finish is not the only thing being changed and the result would not be a fair test.

1. Saying refraction is caused by the wave bending

Bending is what you see, not the cause. State the cause explicitly: the wave changes speed when it enters a different substance, and that speed change makes it change direction.

2. Claiming frequency changes during refraction

Frequency stays the same when a wave refracts. It is the speed and the wavelength that change. Getting this wrong leads to wrong reasoning with $v=f\lambda$.

3. Saying gamma rays come from electrons or from circuits

Gamma rays come from the nucleus of an atom. Radio waves are the ones produced by oscillations in electrical circuits. Keep the two origins separate.

4. Forgetting the unit of radiation dose

Radiation dose is measured in sieverts (Sv), and it measures the risk of harm, not simply the amount of radiation. A larger dose means greater risk.

5. Not controlling variables in the Leslie cube practical

The distance from each face to the detector and the water temperature must be kept the same. If they vary, you cannot tell whether a difference in reading is due to the surface or to the changed conditions.