5.20–5.21Properties of electromagnetic waves 2
Changes in atoms and the nuclei of atoms can result in electromagnetic waves being generated or absorbed over a wide frequency range. Gamma rays originate from changes in the nucleus of an atom.
Ultraviolet waves, X-rays and gamma rays can have hazardous effects on human body tissue. The effects depend on the type of radiation and the size of the dose.
Radiation dose, in sieverts, is a measure of the risk of harm resulting from an exposure of the body to the radiation. 1000 millisieverts (mSv) = 1 sievert (Sv). You will not be required to recall the unit of radiation dose.
Ultraviolet waves can cause skin to age prematurely and increase the risk of skin cancer. X-rays and gamma rays are ionising radiation that can cause the mutation of genes and cancer.
You should be able to draw conclusions from given data about the risks and consequences of exposure to radiation. That means using the figures — comparing a dose with background levels, or comparing two occupations — rather than stating in general terms that radiation is dangerous.
Worked example
A chest X-ray gives a dose of 0.014 mSv. A CT scan gives 6.6 mSv. Average annual background is 2.7 mSv. Draw conclusions about the risks.
- Compare the X-ray with background: 0.014 ÷ 2.7 is about 0.005, so roughly two days' worth of background radiation.
- Compare the CT scan with background: 6.6 ÷ 2.7 is about 2.4 years' worth.
- So the CT scan gives roughly 470 times the dose of the chest X-ray.
- Both are ionising, so both carry some risk of gene mutation and cancer, but the risk from the chest X-ray is very small compared with everyday exposure.
- Conclude in terms of whether the benefit justifies each dose.
Answer: The chest X-ray dose is comparable to about two days of natural background, so the added risk is negligible. The CT scan is about 2.4 years' worth, so its much larger dose needs a correspondingly stronger clinical reason.
5.22Uses and applications of electromagnetic waves
Electromagnetic waves have many practical applications. These are the examples named in the specification, and they are worth learning as a list.
Higher tier students should be able to give brief explanations why each type of electromagnetic wave is suitable for the practical application.
The explanation always comes from a property, so pair each use with the reason:
• Radio waves are not strongly absorbed by the atmosphere and can be reflected or diffracted around obstacles, so they travel long distances to aerials. • Microwaves pass through the atmosphere, so they reach satellites; a different wavelength is absorbed by water molecules in food, which heats it. • Infrared is strongly absorbed by surfaces and transfers energy to their thermal store, which is why it heats and cooks; and because all warm objects emit it, a camera can detect it. • Visible light is transmitted along glass fibres with very little absorption, so it carries data with little loss. • Ultraviolet makes some materials fluoresce, giving visible light efficiently, and it darkens skin. • X-rays are transmitted by flesh but absorbed by bone and metal, which is what makes the image; gamma rays penetrate deeply enough to reach and destroy tumours inside the body.
Common mistake
A use is not an explanation. "Microwaves are used for satellite communication" scores nothing on a Higher tier explain question — say that they pass through the atmosphere without being absorbed.
5.23Radio waves and electrical circuitsHigher tier
Higher tier: radio waves can be produced by oscillations in electrical circuits.
When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit.
That is transmission and reception described in two sentences, and the symmetry is the point. At the transmitter, an alternating current in a circuit produces radio waves of the same frequency. At the receiver, those radio waves are absorbed by an aerial and produce an alternating current of that same frequency, which the receiver then decodes. The frequency carries through unchanged from one circuit to the other, which is why a radio can be tuned to a station.
Before you practise
- Pair every use with the property behind it, not just the name of the wave.
- Radio for broadcasting and communications; microwaves for satellite communications and cooking; infrared for heating, cooking and cameras.
- Visible for fibre optic communication and photography; ultraviolet for security marking and sun tanning; X-rays and gamma for imaging and treating cancer.
- Danger increases with frequency, so ultraviolet, X-rays and gamma rays are the ionising ones.
- Ultraviolet damages skin and can cause skin cancer and eye damage; X-rays and gamma rays are ionising and can cause mutations.
- Microwaves are absorbed by water in tissue and can heat it; infrared causes skin burns.
- Radio waves are produced by oscillations in an electrical circuit, and can induce oscillations in a receiving circuit.