Dangers of Electromagnetic Waves

The same electromagnetic waves that bring us so many benefits can also be harmful, and in general the higher the frequency, the more dangerous the wave. A low-frequency radio wave passes through you with no ill effect, but high-frequency ultraviolet, X-rays and gamma rays carry enough energy to damage the cells of your body — even to alter the DNA inside them and cause cancer. Understanding which waves are dangerous, and why, lets us use them safely: a radiographer steps behind a screen before taking an X-ray, and we wear sunscreen against ultraviolet. This lesson, part of Topic P5 (Waves in matter) of OCR Gateway Science A, sets out the dangers of each band, explains why higher-frequency (ionising) radiation is more harmful, introduces radiation dose, and looks at how radio waves are produced by oscillating charges.

By the end of this lesson you should be able to describe the dangers of microwave, infrared, ultraviolet, X-ray and gamma radiation, explain why higher-frequency ionising radiation is more harmful, describe radiation dose qualitatively, and explain how radio waves are produced by and induce oscillations of charge in an aerial.

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Why Higher Frequency Means More Danger

The danger of an electromagnetic wave depends on how much energy it carries, and the higher the frequency, the higher the energy. Recall that across the spectrum from radio to gamma, the frequency (and energy) increases. So the most harmful EM waves are the high-frequency ones at the gamma end, and the least harmful are the low-frequency radio waves.

The crucial dividing line is whether a wave is ionising. Ionising radiation carries enough energy to knock electrons off atoms, turning them into charged particles called ions. The three highest-energy groups — ultraviolet, X-rays and gamma rays — are ionising (UV is the weakest of these). Ionising radiation is dangerous because, inside the body, it can:

• damage or kill living cells;
• mutate the DNA inside cells (change the genetic code);
• cause cells to grow out of control, leading to cancer.

Lower-frequency waves (radio, microwave, infrared, visible) are not ionising — they do not carry enough energy to knock electrons off atoms — so their main hazard is simply heating, which is far less serious than ionisation.

Exam Tip: The key principle is: higher frequency → higher energy → more dangerous. The three ionising EM waves are ultraviolet, X-rays and gamma rays — they can damage cells and mutate DNA, causing cancer. Lower-frequency waves only heat.

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Dangers Band by Band

Microwaves. Microwaves are absorbed by water, and your body is largely water, so high-intensity microwaves can cause internal heating of body tissues — heating you from the inside. This is why microwave ovens are shielded to keep the microwaves in. (At everyday levels, such as from mobile phones, the heating is very small.)

Infrared. Infrared is absorbed by the skin and felt as heat. Too much infrared causes skin burns — the same way you can be burned by touching something hot or sitting too close to a fire or heater. The damage is thermal (heating), not ionising.

Ultraviolet. Ultraviolet is ionising, and overexposure (typically from the Sun or sun-beds) can cause:

• skin cancer (by damaging the DNA in skin cells);
• premature ageing of the skin (wrinkles, sun damage);
• eye damage, including damage to the retina, if you look at strong UV sources.

This is why sunscreen and sunglasses are recommended, and why sun-beds carry health warnings.

X-rays and gamma rays. These are the most dangerous, being highly ionising and very penetrating. They can:

• damage or kill cells;
• mutate DNA, causing cancer.

A single medical X-ray gives only a small, carefully controlled dose, but repeated or high exposure is harmful, which is why radiographers stand behind a lead screen and limit how often scans are taken.

EM wave	Main danger	Ionising?
Microwave	Internal heating of body tissue	No (heating only)
Infrared	Skin burns	No (heating only)
Ultraviolet	Skin cancer, premature ageing, eye damage	Yes
X-ray	Cell damage, DNA mutation, cancer	Yes
Gamma ray	Cell damage, DNA mutation, cancer	Yes

Exam Tip: Match the danger to the band. Microwave → internal heating; infrared → skin burns; ultraviolet → skin cancer and eye damage; X-ray and gamma → cell damage, DNA mutation and cancer (ionising). Naming the specific harm, not just "it's dangerous", scores the mark.

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Radiation Dose

When we talk about how much harm ionising radiation might do to a person, we use the idea of radiation dose — a measure of the risk of harm to body tissue from being exposed to radiation. (Its unit is the sievert, Sv, but for OCR you only need a qualitative understanding.)

The key points are:

• a higher dose means a greater risk of harm (such as developing cancer);
• the dose depends on how much radiation you are exposed to and for how long, and on how harmful that type of radiation is;
• different activities give different doses — for example, a chest X-ray gives a small dose, while a CT scan gives a larger one, and people who work with radiation wear badges to monitor the dose they receive.

Doctors and radiographers weigh the risk from the radiation dose against the benefit of the procedure: a single X-ray to diagnose a broken bone is well worth its tiny risk, but unnecessary repeated exposure is avoided.

Exam Tip: Radiation dose measures the risk of harm from radiation exposure. The bigger the dose, the greater the risk. You do not need numbers for OCR — just that more exposure (or more harmful radiation) gives a higher dose and so a higher risk.

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How Radio Waves Are Produced and Detected

Radio waves provide a neat example of how electromagnetic waves are produced by moving charges — and how, in turn, they can make charges move.

Producing radio waves. Radio waves are produced by oscillations of charge — alternating currents — in an electrical conductor called an aerial (antenna). When an alternating current flows in the aerial, the electrons (charges) oscillate back and forth, and these oscillating charges emit radio waves at the same frequency as the oscillation. The faster the charges oscillate (the higher the frequency of the alternating current), the higher the frequency of the radio waves produced.

Detecting radio waves. The process also works in reverse. When radio waves arrive at a receiving aerial, they make the electrons in it oscillate — the waves induce an alternating current in the aerial, at the same frequency as the radio waves. This tiny alternating current is the signal that a radio or television then amplifies and turns into sound or pictures.

So there is a pleasing symmetry: oscillating charges in a transmitting aerial produce radio waves, and radio waves induce oscillating charges (an alternating current) in a receiving aerial. The frequency is preserved throughout — the radio waves have the same frequency as the charges that made them, and they make the receiving charges oscillate at that same frequency.