Answers · Light and the electromagnetic spectrum

The electromagnetic spectrum at work — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. UQ14 · Practice

Model answer. All electromagnetic waves, from radio waves to gamma rays, travel through a vacuum at the same high speed, the speed of light. Their frequencies and wavelengths are very different: radio waves have long wavelengths and low frequencies, gamma rays very short wavelengths and very high frequencies.

A full answer includes

Their speed.

Also accept

speed

Watch for

“Their frequency.” — The frequency changes from region to region: it is lowest for radio waves and highest for gamma rays.
“Their wavelength.” — The wavelength changes from region to region: radio waves can be metres long, gamma rays far smaller than an atom.

2. UQ15 · Practice

Model answer. X-rays and radio waves are both electromagnetic waves, and in a vacuum all electromagnetic waves travel at the same speed. Leaving at the same moment and crossing the same empty space, the two signals arrive together. X-rays have a much higher frequency and a much shorter wavelength, but not a higher speed.

A full answer includes

They arrive together: all electromagnetic waves travel at the same speed in a vacuum.

Also accept

together

Watch for

“The X-rays arrive first, because they have the higher frequency.” — A higher frequency means a shorter wavelength, not a higher speed. In a vacuum every region travels at the same speed.
“The radio signal arrives first, because radio waves are longer and cover more distance with each wave.” — Longer waves come with a lower frequency: fewer waves each second. The speed is the same.

3. UQ16 · Practice

Model answer. X-rays and visible light are both electromagnetic waves, and all electromagnetic waves travel at the same speed in a vacuum. X-rays pass through skin and muscle because those materials absorb very little of them, while they absorb or scatter visible light; it is about how matter absorbs each region, not about speed. X-rays do differ from visible light in their frequency, which is much higher, and their wavelength, which is much shorter.

A full answer includes

In a vacuum X-rays and visible light travel at the same speed; both are electromagnetic waves.
Whether a wave passes through the body depends on how the body absorbs it, not on its speed.
X-rays have a higher frequency and a shorter wavelength than visible light.

Also accept

same speed; passing through depends on absorption; higher frequency and shorter wavelength

Watch for

“X-rays are faster because they have more energy.” — Every region travels at the same speed in a vacuum, whatever it can do to matter.
“X-rays have a longer wavelength than light, so they pass through.” — X-rays have a much shorter wavelength than visible light.

4. UQ17 · Practice

Model answer. Radio waves are electromagnetic waves, so in the vacuum of space they travel at 3.0 × 10⁸ m/s. Time = distance ÷ speed = 3.9 × 10⁸ m ÷ 3.0 × 10⁸ m/s = 1.3 s.

Checked automatically

Time: 1.3 s (within ±0.05 s)
- Common wrong answer 0.77 : You divided the speed by the distance. Time = distance ÷ speed.

A full answer includes

Time = 3.9 × 10⁸ m ÷ 3.0 × 10⁸ m/s = 1.3 s.

Also accept

1.3 s

Watch for

“Radio waves are slower than light, so it takes longer.” — All electromagnetic waves, radio waves included, travel at 3.0 × 10⁸ m/s in a vacuum.
“Time = distance × speed.” — Time = distance ÷ speed. Multiplying gives a huge number with the wrong unit.

5. UQ18 · Practice

Model answer. (a) Satellite television uses microwaves, which are electromagnetic waves and travel at 3.0 × 10⁸ m/s. Time = distance ÷ speed = 3.6 × 10⁷ m ÷ 3.0 × 10⁸ m/s = 0.12 s. (b) In air the speed of electromagnetic waves is approximately the same as in a vacuum, only a tiny fraction slower, so the answer to (a) hardly changes.

Checked automatically

(a) Time: 0.12 s (within ±0.005 s)
- Common wrong answer 1.2 : Check the powers of ten: 10⁷ ÷ 10⁸ is 0.1.
- Common wrong answer 8.3 : You divided the speed by the distance. Time = distance ÷ speed.

A full answer includes

(a) Time = 3.6 × 10⁷ m ÷ 3.0 × 10⁸ m/s = 0.12 s.
(b) Approximately the same: only a tiny fraction slower.

Also accept

(a) 0.12 s (b) approximately the same

Watch for

“In air the signal is much slower, like sound.” — Electromagnetic waves in air travel at approximately the same speed as in a vacuum.
“The time is 1.2 s.” — Check the powers of ten: 3.6 × 10⁷ is a tenth of 3.6 × 10⁸.

6. UQ19 · Practice

Model answer. Electromagnetic waves travel at 3.0 × 10⁸ m/s in a vacuum, and at approximately the same speed in air. Glass slows light much more, to about two thirds of that, which is why glass refracts it.

A full answer includes

3.0 × 10⁸ m/s.

Also accept

3.0 × 10⁸ m/s

Watch for

“About 340 m/s.” — That is roughly the speed of sound in air, a million times slower than light.
“3.0 × 10⁸ km/s.” — The speed is 3.0 × 10⁸ metres per second, which is 300 000 kilometres per second.

7. UQ20 · Practice

Options as numbered on the pupil sheet: 1 — Infrared; 2 — X-rays; 3 — Radio waves; 4 — Gamma rays; 5 — Ultraviolet; 6 — Microwaves.

Model answer. A travel card’s RFID tag answers a reader with radio waves. Mobile phones talk to the network with microwaves. A thermal imaging camera detects the infrared that warm bodies give out. A bank note checker shines ultraviolet, which makes the special ink on real notes glow. An airport scanner sends X-rays through bags, which metal objects absorb. Gamma rays pass through the packaging and kill the bacteria on medical equipment already sealed inside.

A full answer includes

RFID tag in a travel card: radio waves.
Mobile phone: microwaves.
Thermal imaging: infrared.
Detecting fake bank notes: ultraviolet.
Airport security scanner: X-rays.
Sterilising sealed medical equipment: gamma rays.

Also accept

RFID radio; phone microwaves; thermal imaging infrared; bank notes ultraviolet; bags X-rays; sterilising gamma rays

Watch for

“A thermal imaging camera uses ultraviolet.” — Warm bodies give out infrared, and that is what a thermal camera detects.
“Medical equipment is sterilised with X-rays.” — The syllabus use is gamma rays: they pass through the packaging and kill the bacteria.

8. UQ21 · Practice

Model answer. A TV remote flashes a code in infrared, a short-range signal. Your eyes cannot detect infrared, so the remote looks dark, but the detector on the television, and the camera of a phone, can. Infrared does not pass through walls, so the remote does not work the television in the next room.

A full answer includes

Infrared.

Also accept

infrared

Watch for

“Radio waves, because it works without a wire.” — Radio waves would pass through the walls into the next room. A TV remote uses infrared, which stays in the room.
“Visible light, because there is a small lamp at the front.” — The lamp at the front gives out infrared; that is why it looks dark to your eyes.

9. UQ22 · Practice

Model answer. (a) Satellite television uses microwaves, which pass through the atmosphere and clouds to the satellite and back. (b) Drinking water is sterilised with ultraviolet, which kills bacteria and viruses. (c) An intruder alarm detects the infrared given out by a warm body, which works in the dark. (d) A camera takes a photograph with visible light, which its sensor detects.

A full answer includes

(a) Microwaves: they pass through the atmosphere and clouds.
(b) Ultraviolet: it kills bacteria and viruses.
(c) Infrared: a warm body gives it out, and the detector senses it, even in the dark.
(d) Visible light: the camera’s sensor detects it.

Also accept

(a) microwaves (b) ultraviolet (c) infrared (d) visible light

Watch for

“Radio waves for satellite television, because television uses radio.” — Ground transmitters use radio waves, but satellite television uses microwaves.
“Gamma rays for drinking water.” — Gamma rays sterilise food and medical equipment. Drinking water is sterilised with ultraviolet.

10. UQ23 · Practice

Model answer. (a) Q, behind the bone. (b) X-rays pass through soft materials such as skin and muscle, so plenty reach P, and at R they miss the arm altogether. Bone absorbs X-rays, so far fewer reach Q. Where fewer X-rays arrive, the picture stays pale, so the bone shows as a pale shadow on a dark background.

A full answer includes

(a) Q.
(b) Skin and muscle let X-rays through, but bone absorbs them, so fewer arrive behind the bone; the bone shows as a pale shadow.

Also accept

(a) Q (b) bone absorbs X-rays; soft tissue lets them through

Watch for

“R, because the X-rays miss the arm there.” — Missing the arm means nothing absorbs them, so the most X-rays arrive at R.
“The bone shows because it reflects the X-rays back to the machine.” — Bone absorbs X-rays; it does not reflect them. Fewer reach the detector behind it.

11. UQ24 · Practice

Options as numbered on the pupil sheet: 1 — Damage to surface cells and eyes, leading to skin cancer and eye conditions; 2 — Internal heating of body cells; 3 — Mutation or damage to cells in the body; 4 — Skin burns.

Model answer. Microwaves pass into the body and are absorbed by its water, heating body cells from inside. Infrared is absorbed by the skin and too much causes skin burns. Ultraviolet damages the surface cells of the skin and the eyes, which can lead to skin cancer and eye conditions such as cataracts. X-rays and gamma rays pass deep into the body and can cause mutation of, or damage to, its cells.

A full answer includes

Microwaves: internal heating of body cells.
Infrared: skin burns.
Ultraviolet: damage to surface cells and eyes, leading to skin cancer and eye conditions.
X-rays and gamma rays: mutation or damage to cells in the body.

Also accept

microwaves internal heating; infrared skin burns; ultraviolet surface cells and eyes; X-rays and gamma rays mutation or cell damage

Watch for

“Infrared causes skin cancer.” — Too much infrared burns the skin. Skin cancer is linked with too much ultraviolet.
“X-rays only harm the skin, because they pass through.” — X-rays pass deep into the body, and on the way they can damage cells or cause mutation.

12. UQ25 · Practice

Model answer. The metal mesh reflects microwaves back into the oven, and the oven switches off when the door opens, so the microwaves stay inside. Too many microwaves would be absorbed by the water in the cook’s body and cause internal heating of body cells.

A full answer includes

To keep the microwaves inside: they would heat the cook’s body cells internally.

Also accept

keeps microwaves in; they would heat body cells internally

Watch for

“To keep the heat of the food inside.” — The mesh is there to keep the microwaves in, because they would heat the cook’s body cells.
“Because microwaves cause skin cancer.” — The harm of too many microwaves is internal heating of body cells.

13. UQ26 · Practice

Model answer. (a) Ultraviolet damages the surface cells of the skin and the eyes. (b) Over the years, too much can lead to skin cancer and to eye conditions such as cataracts. (c) The warmth the gardener feels is infrared. Ultraviolet cannot be felt at all, so feeling only warmth is no guide to the harm being done. (d) A hat, sunscreen and sunglasses that block ultraviolet protect the skin and eyes, and so does spending less time in strong sunlight.

A full answer includes

(a) Ultraviolet.
(b) Skin cancer; eye conditions such as cataracts (damage to surface cells and eyes).
(c) The warmth is infrared; ultraviolet cannot be felt, yet it damages cells.
(d) Any two: a hat, sunscreen, sunglasses that block ultraviolet, less time in strong sunlight.

Also accept

(a) ultraviolet (b) skin cancer; eye conditions (c) ultraviolet cannot be felt (d) e.g. hat, sunscreen

Watch for

“Infrared damages the surface cells and causes skin cancer.” — Too much infrared burns the skin. Damage to surface cells and skin cancer come from ultraviolet.
“If it does not hurt, no harm is being done.” — You cannot feel ultraviolet, yet it damages cells.

14. UQ27 · Practice

Model answer. (a) Gamma rays pass right through the plastic of the packets and kill the bacteria on the spices inside, so the packets stay sealed. (b) Gamma rays pass deep into the body and can cause mutation of, or damage to, its cells, which can lead to cancer. (c) You cannot feel gamma rays at all, so feeling nothing is no guide; they would damage cells without any warning. That is why the room has thick concrete walls and the workers stay outside while it runs.

A full answer includes

(a) Gamma rays pass through the packaging and kill the bacteria inside.
(b) Mutation or damage to cells in the body, which can lead to cancer.
(c) Gamma rays cannot be felt at all, yet they damage the cells they pass through; so the thick walls keep them in.

Also accept

(a) pass through packaging; kill bacteria (b) mutation or cell damage (c) they cannot be felt

Watch for

“Gamma rays sterilise by heating the spices.” — They kill the bacteria directly as they pass through; the spices are not cooked.
“Gamma rays would feel warm, like infrared.” — You cannot feel gamma rays at all.

15. UT2 · Apply

Model answer. (a) Waves longer than microwaves are radio waves, which the dish of a radio telescope receives: astronomy is one use of radio waves. On the Earth radio waves are also used for radio and television transmissions and for RFID tags. (b) Radio waves and light are both electromagnetic waves, so in empty space they travel at the same speed. (c) Time = distance ÷ speed = 6.3 × 10¹¹ m ÷ 3.0 × 10⁸ m/s = 2100 s, which is 35 minutes. (d) Too much ultraviolet damages the surface cells of the skin and the eyes, which can lead to skin cancer and eye conditions such as cataracts. X-rays pass deep into the body and can cause mutation of, or damage to, its cells.

Checked automatically

(c) Supplement: Time for the waves to arrive: 2100 s (within ±10 s)
- Common wrong answer 210 : Check the powers of ten: 10¹¹ ÷ 10⁸ is 1000.
- Common wrong answer 35 : That is the time in minutes. The answer box is in seconds.

A full answer includes

(a) Radio waves; for example radio and television transmissions or RFID tags.
(b) The same speed: all electromagnetic waves travel at the same speed in a vacuum.
(c) Supplement: 6.3 × 10¹¹ m ÷ 3.0 × 10⁸ m/s = 2100 s (35 minutes).
(d) Ultraviolet: damage to surface cells and eyes, leading to skin cancer and eye conditions; X-rays: mutation or damage to cells in the body.

Also accept

(a) radio waves; e.g. radio and television (b) the same (c) 2100 s (d) ultraviolet: surface cells and eyes; X-rays: mutation or cell damage

Watch for

“Light is faster than the radio waves.” — All electromagnetic waves travel at the same speed in a vacuum.
“The waves take 2100 minutes.” — Distance ÷ speed in m and m/s gives seconds: 2100 s, which is 35 minutes.