Chapter 8 of 8

All 8 chapters
  1. 1Kinds of waves, readunderstood
  2. 2Measuring waves, readunderstood
  3. 3Waves on water, readunderstood
  4. 4Sound and ultrasound, readunderstood
  5. 5Reflection and refraction, readunderstood
  6. 6Lenses, readunderstood
  7. 7The electromagnetic spectrum, readunderstood
  8. 8Waves in communication and health, readunderstood

S4.10.8 Chapter 8

Waves in communication and health

Amina calls her cousin across the sea, and her voice arrives in a fraction of a second. What carries it?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how radio, microwaves and light carry phone and internet signals
  • Explain why ultraviolet, X-rays and gamma rays can harm living cells

A wave can carry energy. By changing the wave in a pattern, it can also carry a message. The plain wave that is changed is the carrier wave. A radio station, a mobile phone and a fibre cable all work this way.

In an analogue signal, the changes copy the message smoothly. In amplitude modulation the height of the carrier follows the message. In frequency modulation the frequency of the carrier follows it.

Two waves one above the other. The top wave is a slow smooth wave labelled message. The wave below it is a much faster wave labelled wave sent, whose height rises and falls in step with the top wave.messagewave sent
Amplitude modulation. The height of the fast wave follows the message.
Two waves one above the other. The top wave is a slow smooth wave labelled message. The wave below it, labelled wave sent, keeps one height. Its waves bunch up where the message is high and spread out where it is low.messagewave sent
Frequency modulation. The frequency of the fast wave follows the message.

Phone calls and the internet use digital signals. In a digital signal the message is turned into numbers, written with only two digits, 1 and 0. Each digit is a bit. A bit is sent as a pulse, high for 1 and low for 0.

Two rows of pulses one above the other. The top row is labelled bits, with the digits 1 0 1 1 0 1 0 0 above it and a high step for each 1 and a low step for each 0. The row below is labelled pulses sent and has the same high and low steps.10110100bitspulses sent
A digital signal. Only two levels are used, so each bit can be told from the others.

Digital signals have an advantage on a long journey. A wave gets weaker and picks up noise on the way. A digital signal has only two levels. So even a weak, noisy pulse can be read as a 1 or a 0. It can be cleaned up and sent on as a fresh pulse. An analogue signal cannot be cleaned, because its noise looks like part of the message.

Many senders share the air. Each uses its own frequency. A radio set tunes to one frequency, which picks out one station and ignores the rest. Mobile phones also share the air by using different frequencies.

Now follow a call. The phone sends its code as microwaves to a mast. The mast passes it on, mostly as pulses of light in a glass optical fibre cable. The light is kept in by total internal reflection, which you met in the chapter on reflection and refraction. The fibre carries it over land and under the sea.

Four things one above the next, joined by arrows. The first is labelled phone, then mast, then far mast, then far phone. The arrow between the phone and the mast is labelled microwaves. The arrow between the mast and the far mast is labelled light in fibre. The arrow between the far mast and the far phone is labelled microwaves.phonemicrowavesmastlight in fibrefar mastmicrowavesfar phone
One way a call crosses a sea. The fibre between the masts runs over land and under the sea. The kind of wave changes along the way.

Light in a fibre is usually infrared, not visible light. A fibre can carry far more information than a copper wire, loses little energy, and is not disturbed by electrical interference. Most of the internet between cities and across oceans travels this way.

Microwaves pass through the atmosphere, so dishes on the ground can send them to a satellite and back. They travel in straight lines, so two dishes on hills need a clear view of each other. Wi-Fi also uses microwaves, to link a router to a phone or a computer.

A high layer of the air reflects some long radio waves. They can then travel round the curve of the Earth. This is why some radio stations can be heard from far away. Light and microwaves are not reflected in this way.

All of these signals travel at the speed of the wave. In a vacuum, microwaves and radio waves travel at 3 × 10⁸ m/s. Light in glass fibre is slower, about 2 × 10⁸ m/s. The time a signal takes is the distance divided by the speed.

Worked example

A signal crosses an undersea fibre cable 8 000 km long. Light in the fibre travels at 2 × 10⁸ m/s. How long does the signal take?

  1. Change the distance to metres: 8 000 km = 8 000 000 m.
  2. The time is t = d ÷ v = 8 000 000 ÷ 200 000 000 = 0.04 s.

Answer: 0.04 s

Two things one above the other, joined by an arrow. The first is labelled dish and the second is labelled dish. The arrow between them is labelled microwaves, with 60 km under it.dishmicrowaves60 kmdish
Two hilltop dishes linked by microwaves, 60 km apart.

Worked example

Two hilltop dishes are 60 km apart. Microwaves travel between them at 3 × 10⁸ m/s. How long does a signal take?

  1. Change the distance to metres: 60 km = 60 000 m.
  2. The time is t = d ÷ c = 60 000 ÷ 300 000 000 = 0.0002 s.
  3. That is 0.2 ms, much shorter than a blink.

Answer: 0.0002 s

Now think about health. When an electromagnetic wave is absorbed, it hands over energy. It does this in tiny packets. The energy of one packet is higher for a higher frequency. A wave with twice the frequency has packets with twice the energy.

Radio waves, microwaves, infrared and visible light have packets with too little energy to knock electrons off atoms. Their main effect is heating. Strong microwaves heat water and can burn, so a microwave oven is shielded and its door has a safety switch. Strong infrared also burns.

Ultraviolet, X-rays and gamma rays have packets with much more energy. The shortest ultraviolet, and all X-rays and gamma rays, can knock electrons off atoms. This is called ionising. They are ionising radiation. Even the longer ultraviolet can harm a cell by changing its chemistry.

An atom as a dot with a dashed ring and two electrons on it. A wavy ray labelled ray arrives from the left. Short double arrows beside the electrons show that they only shake. The atom is labelled atom and one electron is labelled electron. Nothing leaves the atom.rayatomelectron
A ray with low energy packets only makes the atom shake. It warms.
An atom as a dot with a dashed ring, a plus sign beside it and one electron on the ring. A wavy ray labelled ray arrives from the left. A second electron, labelled electron, has been knocked out and moves away along an arrow. The atom is labelled atom and the changed atom is labelled ion.+rayatomelectronion
A ray with high energy packets knocks an electron out. The atom becomes an ion.

Inside a living cell, the nucleus holds the DNA, which is the instruction for the cell. Ionising radiation can damage DNA. A cell can often repair small damage. A badly damaged cell dies. Sometimes a damaged cell lives on and copies its changed DNA. A change in DNA is a mutation.

If a mutation falls in the wrong place, the cell may grow and divide out of control. This is cancer. The more ionising radiation a body takes in, the dose, the greater the chance of harm. A very large dose can cause burns and radiation sickness, and can kill.

Seven rows in order from the top: radio, over 1 m. Microwave, 1 mm to 1 m. Infrared, 700 nm to 1 mm. Visible, 400 to 700 nm. Ultraviolet, 10 to 400 nm. X-ray, 0.01 to 10 nm. Gamma, under 0.01 nm. A thick box is drawn round the last three rows, ultraviolet, X-ray and gamma. A long arrow down the left is labelled frequency rises. A long arrow down the right is labelled wavelength falls.frequency riseswavelength fallsradioover 1 mmicrowave1 mm to 1 minfrared700 nm to 1 mmvisible400 to 700 nmultraviolet10 to 400 nmX-ray0.01 to 10 nmgammaunder 0.01 nm
The boxed waves can damage living cells. The frequency, and so the energy of each packet, rises down the list.

Ultraviolet from the Sun causes sunburn, ages the skin and harms the eyes. It is the main cause of skin cancer. A little sunlight on the skin helps the body make vitamin D, but a short time is enough. Darker skin has more of the pigment that absorbs ultraviolet, which gives some protection but not complete protection.

To protect your skin, stay in the shade near midday. Cover up with clothes and a hat. Wear sunglasses that block ultraviolet, and use sun cream. Thin cloud does not stop ultraviolet, so you can burn on a cloudy day.

X-rays are useful. A doctor orders one only when the benefit is greater than the small risk. The dose from one dental X-ray is small. The risk is small, but it is not zero. So staff stand behind a screen or leave the room. A lead apron may cover the parts of the body that are not being pictured. A woman who might be pregnant must tell the staff first.

Gamma rays kill cells too, which is why a narrow beam aimed from several directions treats cancer. The tumour gets a large dose and the tissue around it gets much less. Strong sources are kept in thick lead or concrete, and workers limit their time near them.

For people near ionising radiation, three rules help. Spend less time near it, stay further away, and put something thick between you and it, such as lead or concrete. Each one lowers the dose.

What about mobile phones and Wi-Fi? Their microwaves cannot ionise atoms. Their main effect on the body is a very small warming. Large studies have not found that phones cause harm. A cancer research agency of the World Health Organization lists radio frequency fields as "possibly" cancer causing. It uses that group when the evidence is limited. Studies go on.

One more danger is not about packets but about heat. A very bright beam of visible light can burn the retina at the back of the eye. Never look at the Sun, and never look into a laser beam, even for a moment. The damage can be permanent.

Try it

Send a message with a torch

You need: a torch, a friend at the other side of a dim room, or across a dark yard, paper and a pencil for each of you.

Be safe: Never shine the torch into anyone's eyes, and never use a laser pointer for this. Do it where nobody can trip in the dim light, and not near roads.

  1. Agree a code of two flashes for each letter. A is short short, B is short long, C is long short and D is long long. A short flash is a 0 and a long flash is a 1.
  2. Write a message of four letters, using only A, B, C and D. Do not show it to your friend.
  3. Send each letter by two flashes, with a pause between letters. Your friend writes down short or long for each flash.
  4. Your friend uses the code to read the letters. Compare with your message.
  5. Now hold a sheet of paper in front of the torch so that it is dimmer. Send the message again.

What you will see: The message is still read correctly when the torch is dim. Your friend only has to tell a short flash from a long one. A digital signal can be read even when it is weak.

If you cannot try it, this is what happens.
A table with the columns Letter and Flashes. Row 1: A, short short. Row 2: B, short long. Row 3: C, long short. Row 4: D, long long.LetterFlashesAshort shortBshort longClong shortDlong long

A wrong idea many people have

All electromagnetic waves are equally dangerous, or all are safe.

Why it feels right: The word "radiation" is used for all of them, and news stories treat it as one thing.

The test that breaks it: Infrared from a fire warms you, and visible light lets you see. Neither ionises atoms. A dental X-ray is a short exposure to a high energy wave. They are not the same kind of risk.

What is true: What a wave can do depends on its frequency and on the dose. Ultraviolet, X-rays and gamma rays can damage cells. Radio waves, microwaves, infrared and visible light cannot ionise, though strong ones can heat and burn.

A wrong idea many people have

A very small dose of X-rays does no harm at all.

Why it feels right: A dental or chest X-ray is safe enough for doctors to use, so it seems to have no risk.

The test that breaks it: Each packet of an X-ray can damage a molecule of DNA, and one change can start a cancer. The chance is very small for one X-ray, but it is not zero, and it grows with every exposure.

What is true: The risk from a small dose is small, not zero. Doctors use X-rays when the benefit is clearly greater, and they keep the dose as low as they can.

A wrong idea many people have

A strong enough radio signal would damage DNA like an X-ray.

Why it feels right: More power seems always to mean more danger, and "strong" sounds like "high energy".

The test that breaks it: Each radio packet has far too little energy to knock an electron off an atom. Making the beam stronger sends more packets, not bigger ones, so it cannot ionise.

What is true: A strong beam of radio waves or microwaves can heat and burn, but it cannot ionise. Only high frequency waves, with packets of high energy, ionise.

Key words

carrier wave
the plain wave that is changed to carry a message.
analogue signal
a signal in which the wave changes smoothly to copy the message.
digital signal
a signal sent as a code of 1s and 0s, as pulses.
bit
one binary digit, a 1 or a 0.
ionising radiation
waves or particles with enough energy to knock electrons off atoms. It can damage DNA.
mutation
a change in the DNA of a cell.
dose
how much ionising radiation a body takes in.

Back to your guess

The answer is A: Radio waves, microwaves and pulses of light in cables carry it.

The phone turns her voice into a code. Microwaves carry the code to a mast. Pulses of light in glass fibres carry it across land and under the sea. Sound does not travel all the way.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. How does a radio wave carry a message?

    Tap your answer.

    1. Not this one. Sound cannot travel inside an electromagnetic wave. The wave is changed in a pattern that copies the message, and the radio set turns the pattern back into sound. Try another.

    2. Not this one. The speed of a radio wave does not change. It is the height or the frequency that changes. Try another.

    3. Not this one. A radio wave stays a radio wave. The message is put on it by changing the wave. Try another.

    4. Yes, that is right. A carrier wave is changed in step with the message. In amplitude modulation its height follows the message, and in frequency modulation its frequency follows it.

    See the answer

    The wave is changed in step with the message, in its height or frequency.

    A carrier wave is changed in step with the message. In amplitude modulation its height follows the message, and in frequency modulation its frequency follows it.

    • If you chose A: Sound cannot travel inside an electromagnetic wave. The wave is changed in a pattern that copies the message, and the radio set turns the pattern back into sound.
    • If you chose B: The speed of a radio wave does not change. It is the height or the frequency that changes.
    • If you chose C: A radio wave stays a radio wave. The message is put on it by changing the wave.

  2. Why is a digital signal better than an analogue signal over a long distance?

    Tap your answer.

    1. Yes, that is right. A digital signal has only two levels. So a weak or noisy pulse can still be read as a 1 or a 0. It is then sent on as a fresh pulse. An analogue signal cannot be cleaned.

    2. Not this one. Both kinds travel at the same speed. The advantage is that a digital signal can be cleaned up on the way. Try another.

    3. Not this one. Every signal needs a wave or a current to carry it. A digital signal is a pattern of pulses on a wave. Try another.

    4. Not this one. It does get weaker. What helps is that with only two levels, it can be read and renewed before the noise swamps it. Try another.

    See the answer

    A weak, noisy digital signal can still be read as high or low pulses, and cleaned into fresh ones.

    A digital signal has only two levels. So a weak or noisy pulse can still be read as a 1 or a 0. It is then sent on as a fresh pulse. An analogue signal cannot be cleaned.

    • If you chose B: Both kinds travel at the same speed. The advantage is that a digital signal can be cleaned up on the way.
    • If you chose C: Every signal needs a wave or a current to carry it. A digital signal is a pattern of pulses on a wave.
    • If you chose D: It does get weaker. What helps is that with only two levels, it can be read and renewed before the noise swamps it.

  3. How does most of a phone call cross an ocean?

    Tap your answer.

    1. Yes, that is right. The code goes as pulses of light, usually infrared, in optical fibres under the sea. The light is kept inside by total internal reflection.

    2. Not this one. Sound in water travels far less well than a cable, and a phone does not make sound in the sea. Try another.

    3. Not this one. Sea water absorbs radio waves quickly. A cable under the sea does the job. Try another.

    4. Not this one. Old cables used copper wires. Modern ones use glass fibres and light, which carry much more. Try another.

    See the answer

    As pulses of light in glass fibres on the sea floor

    The code goes as pulses of light, usually infrared, in optical fibres under the sea. The light is kept inside by total internal reflection.

    • If you chose B: Sound in water travels far less well than a cable, and a phone does not make sound in the sea.
    • If you chose C: Sea water absorbs radio waves quickly. A cable under the sea does the job.
    • If you chose D: Old cables used copper wires. Modern ones use glass fibres and light, which carry much more.

  4. Which of these can ionise atoms?

    Tap your answer.

    1. Not this one. Infrared packets have too little energy to knock an electron off an atom. They warm things instead. Try another.

    2. Not this one. Radio packets have very little energy. Even a strong radio beam only heats, because it sends more packets, not bigger ones. Try another.

    3. Yes, that is right. Gamma rays have the highest frequency and so packets with the most energy. They can knock electrons off atoms. Infrared and radio waves cannot.

    See the answer

    Gamma rays

    Gamma rays have the highest frequency and so packets with the most energy. They can knock electrons off atoms. Infrared and radio waves cannot.

    • If you chose A: Infrared packets have too little energy to knock an electron off an atom. They warm things instead.
    • If you chose B: Radio packets have very little energy. Even a strong radio beam only heats, because it sends more packets, not bigger ones.

  5. Why can X-rays and gamma rays harm living cells, when radio waves cannot?

    Tap your answer.

    1. Yes, that is right. The energy of one packet rises with frequency. X-ray and gamma packets have enough energy to ionise atoms and damage DNA. Radio packets do not.

    2. Not this one. All electromagnetic waves go at the same speed in a vacuum. It is the energy of each packet that matters, and that rises with frequency. Try another.

    3. Not this one. They have a shorter wavelength. The longer the wavelength, the less the energy of each packet. Try another.

    4. Not this one. More packets each second means more energy in total. Each packet has the same energy, and that depends on the frequency. Try another.

    See the answer

    Each packet carries enough energy to damage DNA.

    The energy of one packet rises with frequency. X-ray and gamma packets have enough energy to ionise atoms and damage DNA. Radio packets do not.

    • If you chose B: All electromagnetic waves go at the same speed in a vacuum. It is the energy of each packet that matters, and that rises with frequency.
    • If you chose C: They have a shorter wavelength. The longer the wavelength, the less the energy of each packet.
    • If you chose D: More packets each second means more energy in total. Each packet has the same energy, and that depends on the frequency.

  6. Which is the best way to protect your skin from ultraviolet in strong sunlight?

    Tap your answer.

    1. Not this one. A tan is a sign of skin that has been damaged. It gives only a little protection. Try another.

    2. Not this one. Thin cloud lets much ultraviolet through, and you can burn. Cloud is not a safe shield. Try another.

    3. Not this one. Ultraviolet damages cells, and the damage builds up. Cover and shade protect the skin. Try another.

    4. Yes, that is right. Shade, clothing, a hat, sunglasses and sun cream all cut the ultraviolet that reaches your skin and eyes. Thin cloud does not stop it.

    See the answer

    Stay in the shade near midday, cover up, and use sun cream.

    Shade, clothing, a hat, sunglasses and sun cream all cut the ultraviolet that reaches your skin and eyes. Thin cloud does not stop it.

    • If you chose A: A tan is a sign of skin that has been damaged. It gives only a little protection.
    • If you chose B: Thin cloud lets much ultraviolet through, and you can burn. Cloud is not a safe shield.
    • If you chose C: Ultraviolet damages cells, and the damage builds up. Cover and shade protect the skin.

  7. A signal goes from a ground dish up to a satellite 36 000 km above, and back down to a second dish. How long does it take? Use c = 3 × 10⁸ m/s.

    Tap your answer.

    1. Yes, that is right. The whole way is 36 000 km up and 36 000 km down. That is 72 000 km. Then t = d ÷ c = 72 000 000 ÷ 300 000 000 = 0.24 s.

    2. Not this one. That is the time to go up only. The signal also has to come down again, so the whole way is 72 000 km. Try another.

    3. Not this one. That is double the time. The whole way is 36 000 km up and 36 000 km down, which is 72 000 km, no more. Try another.

    See the answer

    0.24 s

    The whole way is 36 000 km up and 36 000 km down. That is 72 000 km. Then t = d ÷ c = 72 000 000 ÷ 300 000 000 = 0.24 s.

    • If you chose B: That is the time to go up only. The signal also has to come down again, so the whole way is 72 000 km.
    • If you chose C: That is double the time. The whole way is 36 000 km up and 36 000 km down, which is 72 000 km, no more.

  8. From an earlier chapter

    Microwaves of frequency 3 × 10¹⁰ Hz are used in a radar. What is their wavelength? Use c = 3 × 10⁸ m/s.

    Tap your answer.

    1. Not this one. You divided the frequency by c. The wavelength is c divided by the frequency. Go to S4.10.7. Try another.

    2. Not this one. You multiplied c and f. The wavelength is c divided by f. Go to S4.10.7. Try another.

    3. Not this one. The power of ten is off by one. 3 × 10⁸ divided by 3 × 10¹⁰ is 10⁻², not 10⁻¹. Go to S4.10.7. Try another.

    4. Yes, that is right. λ = c ÷ f = 3 × 10⁸ ÷ (3 × 10¹⁰) = 0.01 m, which is 1 cm.

    See the answer

    0.01 m

    λ = c ÷ f = 3 × 10⁸ ÷ (3 × 10¹⁰) = 0.01 m, which is 1 cm.

    • If you chose A: You divided the frequency by c. The wavelength is c divided by the frequency. Go to S4.10.7.
    • If you chose B: You multiplied c and f. The wavelength is c divided by f. Go to S4.10.7.
    • If you chose C: The power of ten is off by one. 3 × 10⁸ divided by 3 × 10¹⁰ is 10⁻², not 10⁻¹. Go to S4.10.7.

  9. A signal crosses an undersea fibre cable 5 000 km long. Light in the fibre travels at 2 × 10⁸ m/s. How long does the signal take?

    See the answer

    0.025 s

    5 000 km = 5 000 000 m. Then t = d ÷ v = 5 000 000 ÷ 200 000 000 = 0.025 s.

  10. A Wi-Fi router uses microwaves of frequency 2.4 GHz. What is their wavelength in cm? Use c = 3 × 10⁸ m/s and 1 GHz = 10⁹ Hz.

    See the answer

    12.5 cm

    2.4 GHz = 2.4 × 10⁹ Hz. Then λ = c ÷ f = 3 × 10⁸ ÷ (2.4 × 10⁹) = 0.125 m, which is 12.5 cm.

Keep it

Copy this into your exercise book.

A message changes a carrier wave. Digital: pulses of 1 and 0. Calls: radio and microwaves, light in fibres. UV, X-rays and gamma rays ionise and can damage DNA. Dose matters.

Draw: A phone, a mast, a cable under the sea and a second phone, with the kind of wave written on each arrow.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (951 KB). It opens on any phone.