Answers · Communicating with waves

Signals around the world — answer key

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

1. CQ1 · Practice

Model answer. Communication with artificial satellites is mainly by microwaves. Microwaves are electromagnetic waves, so they cross the vacuum of space; they pass through the atmosphere, even through heavy cloud; and a dish can aim them at one satellite in a narrow beam. Sound cannot cross space, heavy cloud blocks visible light, and the long radio waves used for broadcasting across the land would spread out from a dish of this size instead of reaching just the one satellite.

A full answer includes

I chose microwaves.
I did not choose visible light, which the heavy cloud would block.

Also accept

microwaves

Watch for

“Sound waves, because the satellite passes on the voice.” — Sound needs a material to travel through, and space is a vacuum. The satellite receives and sends microwaves that carry the signal.
“Visible light, because light travels fastest.” — All electromagnetic waves travel at the same speed in space. Heavy cloud blocks visible light, but microwaves pass through it.
“Radio waves, because radio is used for broadcasting.” — Long radio waves are used for broadcasting across the land: from a dish of this size they would spread out widely. Satellite links are mainly by microwaves, which the dish can aim at the one satellite.

2. CQ2 · Practice

Options as numbered on the pupil sheet: 1 — A geostationary satellite; 2 — A low-orbit satellite.

Model answer. Satellite P is only a little above the Earth, so it is a low-orbit satellite: it goes round in about an hour and a half and crosses the sky in a few minutes. Satellite Q is high above the equator, so it is a geostationary satellite: it goes round once a day, in step with the turning Earth, and stays above the same place. A dish fixed to a wall can only point at a geostationary satellite, which never moves in the sky.

A full answer includes

I matched satellite P to a low-orbit satellite.
I matched satellite Q to a geostationary satellite.
I matched “takes a whole day to go round the Earth once” to a geostationary satellite.
I matched “crosses the sky in a few minutes” to a low-orbit satellite.
I matched the satellite the fixed dish points at to a geostationary satellite.
I did not match the fixed dish to the low-orbit satellite because it is nearer.

Also accept

P low-orbit; Q geostationary; a whole day to go round geostationary; crosses the sky in minutes low-orbit; fixed dish geostationary

Watch for

“Satellite P is geostationary.” — P is just above the Earth. A geostationary satellite is about 36 000 km up, far above the equator, like Q.
“Satellite Q is a low-orbit satellite.” — Q is high above the equator. Low-orbit satellites are only a few hundred kilometres up, like P.
“A low-orbit satellite takes a whole day to go round.” — A low-orbit satellite goes round in about an hour and a half. Only a geostationary satellite takes one day.
“A geostationary satellite crosses the sky in a few minutes.” — A geostationary satellite never moves in the sky. A low-orbit satellite crosses it in a few minutes.
“The fixed dish points at the low-orbit satellite, because it is nearer.” — A low-orbit satellite crosses the sky in a few minutes, out of the beam of a fixed dish. The dish points at the geostationary satellite, which stays put in the sky.

3. CQ3 · Practice

Options as numbered on the pupil sheet: 1 — Geostationary satellites; 2 — Low-orbit satellites.

Model answer. Near the North Pole, a satellite above the equator is on or below the horizon, but low-orbit satellites pass over the poles. A geostationary satellite is about 36 000 km up, so the signal takes about a quarter of a second each way; low-orbit satellites are only a few hundred kilometres up, so a live video call has no annoying pause. The weak signal from a small phone with a short aerial spreads out as it travels, so it is far stronger when it reaches a satellite a few hundred kilometres up than when it reaches one about 36 000 km away: a weak transmitter works best with satellites close to the Earth. A geostationary satellite is high above the equator, where it is in view from a large part of the Earth and stays above one point, so three of them, spread round the equator, can cover most of the world.

A full answer includes

I matched the hand-held phone near the North Pole to low-orbit satellites.
I matched the live video call to low-orbit satellites.
I matched the pocket-sized phone with a weak transmitter to low-orbit satellites.
I matched the service with only three satellites to geostationary satellites.
I did not match the phone near the North Pole to geostationary satellites.

Also accept

North Pole low-orbit; video call low-orbit; pocket-sized phone low-orbit; only three satellites geostationary

Watch for

“The phone near the North Pole uses a geostationary satellite.” — Near the North Pole, a satellite above the equator is on or below the horizon. Low-orbit satellites pass over the poles.
“The live video call uses a geostationary satellite.” — A geostationary satellite is about 36 000 km up, so the signal takes about a quarter of a second each way. Low-orbit satellites are only a few hundred kilometres up.
“The pocket-sized phone uses a geostationary satellite.” — Its weak signal spreads out as it travels, so it arrives far stronger at a satellite a few hundred kilometres up than at one about 36 000 km away. A weak transmitter works best with the closer, low-orbit satellites.
“Three low-orbit satellites could cover most of the world.” — A low-orbit satellite crosses the sky in minutes, so three could never cover the world all the time. Three geostationary satellites, each staying above one place, can.

4. CQ4 · Practice

Options as numbered on the pupil sheet: 1 — A receiver takes the signal off the microwaves; 2 — The speaker turns the signal back into sound; 3 — A transmitter puts the signal on microwaves and a dish sends them up; 4 — The microphone turns the sound into an electrical signal; 5 — The satellite receives the microwaves and sends them down.

Model answer. The microphone turns the sound of the voice into an electrical signal. A transmitter puts the signal on a carrier, here microwaves, and a dish sends them up. The satellite receives the microwaves and sends them down to Singapore. A receiver there takes the signal off the carrier, and the speaker turns the electrical signal back into sound.

A full answer includes

I put the microphone first.
I put the transmitter and dish second.
I put the satellite third.
I put the receiver fourth.
I put the speaker last.
I did not put the speaker before the receiver.

Also accept

microphone; transmitter; satellite; receiver; speaker

Watch for

“The satellite sends the sound of the voice down to Singapore.” — Sound cannot cross space. The satellite passes on microwaves that carry the signal.
“The speaker comes before the receiver.” — The receiver first takes the signal off the carrier; only then can the speaker turn it into sound.

5. CQ5 · Practice

Model answer. Today the satellite takes one day to go round, the same time the Earth takes to turn once, so it stays above the same point on the equator and the fixed dish keeps pointing at it. A satellite that went round in 12 hours would go round twice while the Earth turns once. It would get ahead of the turning Earth and move across the hotel’s sky, out of the narrow beam of the fixed dish, and the picture would soon be lost. Being above the equator is not enough. A 12-hour orbit is lower, not higher, and the signal would still take only a fraction of a second to come down.

A full answer includes

I chose: the picture would be lost, as the satellite would move across the sky.
I wrote that the satellite would no longer go round in step with the turning Earth.
I wrote that it would not stay above the same point on the equator.
I did not choose “the picture would stay” because the satellite is still above the equator.

Also accept

the picture would be lost: the satellite would no longer keep pace with the turning Earth, so it would move across the sky, out of the fixed dish’s beam

Watch for

“The picture would stay, because the satellite is still above the equator.” — Above the equator is not enough: it must also go round in the time the Earth takes to turn once. In 12 hours it would get ahead and move across the sky.
“The picture would be lost, because the satellite would be further away and its signal too weak.” — A satellite that goes round in 12 hours is closer to the Earth, not further. The problem is that it no longer keeps pace with the turning Earth.
“The picture would be lost, because the signal would take too long to come down.” — The signal takes only a fraction of a second to come down, and a lower orbit makes that shorter. The problem is that the satellite moves out of the dish’s beam.

6. CQ6 · Practice

Model answer. (a) 40 minutes ÷ 9 minutes = 4.4. Four satellites, one after another, carry the call for only 4 × 9 = 36 minutes, so a fifth must carry the last few minutes: 5 satellites. (b) A low-orbit satellite is only a few hundred kilometres up, so it is in view from only a small part of the Earth, and it crosses that part of the sky in minutes. To have one always in view, everywhere, a whole fleet is needed, each passing the call on to the next. A geostationary satellite is about 36 000 km up, so it is in view from a large part of the Earth, and it stays above one point on the equator: three of them, spread round the equator, cover most of the world without ever passing a call on.

Checked automatically

(a) Smallest number of satellites: 5 (exact value)
- Common wrong answer 4 : Four satellites of 9 minutes each carry the call for only 36 minutes. One more is needed.
- Common wrong answer 4.4 : A satellite carries the call for its whole pass or not at all. Round up to a whole number of satellites.

A full answer includes

(a) Method: I divided 40 minutes by 9 minutes: 4.4.
(a) Answer: I wrote 5 satellites.
(a) I did not round down to 4, which carry the call for only 36 minutes.
(b) I wrote that a low-orbit satellite is close to the Earth, so it is in view from only a small part of it.
(b) I wrote that each low-orbit satellite crosses the sky in minutes, so the next one must take over the call.
(b) I wrote that a geostationary satellite is about 36 000 km up, so it is in view from a large part of the Earth.
(b) I wrote that a geostationary satellite stays above one place, so it never has to pass a call on.

Also accept

(a) 5 (b) a low-orbit satellite is in view from only a small area and crosses the sky in minutes; a geostationary one is high, in view from a large part of the Earth, and stays put

Watch for

“(a) Four satellites are enough.” — Four satellites of 9 minutes each carry the call for only 36 minutes. A fifth must carry the last 4 minutes.
“One low-orbit satellite stays above the team all the time.” — Only a geostationary satellite stays above one place. A low-orbit satellite crosses the sky in a few minutes.
“(b) Each low-orbit satellite is weak, so many are needed to make one strong signal.” — A call goes through one satellite at a time. Many are needed because each is in view from only a small area, and only for minutes.

7. CQ7 · Practice

Model answer. (a) Cable: 4 600 km = 4.6 × 10⁶ m, and time = distance ÷ speed = 4.6 × 10⁶ m ÷ 2.0 × 10⁸ m/s = 0.023 s. Satellites: 5 700 km = 5.7 × 10⁶ m, and time = 5.7 × 10⁶ m ÷ 3.0 × 10⁸ m/s = 0.019 s. (b) The satellite route delivers the message first, by about 0.004 s. Its path is 1 100 km longer, but light travels more slowly in glass than microwaves do through space, and the gain in speed more than makes up for the extra distance. The time depends on the speed along a route as well as its length.

Checked automatically

(a) Time by the cable: 0.023 s (2 significant figures)
- Common wrong answer 0.015 : That uses 3.0 × 10⁸ m/s. Light in the glass of the cable travels at 2.0 × 10⁸ m/s.
- Common wrong answer 0.000023 : Change 4 600 km to metres first: 4.6 × 10⁶ m.
- Common wrong answer 43 : Time = distance ÷ speed. You divided the speed by the distance.
(a) Time by the satellites: 0.019 s (2 significant figures)
- Common wrong answer 0.000019 : Change 5 700 km to metres first: 5.7 × 10⁶ m.
- Common wrong answer 53 : Time = distance ÷ speed. You divided the speed by the distance.

A full answer includes

(a) Method: I changed 4 600 km to 4.6 × 10⁶ m and 5 700 km to 5.7 × 10⁶ m.
(a) Method: I used time = distance ÷ speed for each route.
(a) Answer: I wrote 0.023 s for the cable.
(a) Answer: I wrote 0.019 s for the satellites.
(b) I deduced that the satellite route is first.
(b) I wrote that light travels more slowly in glass than microwaves do through space.
(b) I wrote that this gain in speed more than makes up for the longer path.
(a) I did not use 3.0 × 10⁸ m/s for the light in the cable.

Also accept

(a) cable 0.023 s; satellites 0.019 s (b) the satellites: light in glass is slower than microwaves in space, which outweighs the longer path

Watch for

“(b) The cable is first, because its path is shorter.” — Its path is shorter, but light in glass travels at only 2.0 × 10⁸ m/s. Compare the two times you calculated.
“(a) Light in the cable travels at 3.0 × 10⁸ m/s.” — That is its speed in a vacuum. In glass, light is slower: here 2.0 × 10⁸ m/s.
“(b) The satellites are first because microwaves travel faster than light.” — In space, microwaves and light travel at the same speed. The cable’s light is slower because it travels in glass.

8. CQ8 · Practice

Options as numbered on the pupil sheet: 1 — Radio waves; 2 — Visible light or infrared; 3 — Microwaves.

Model answer. Wi-fi and mobile phone networks use microwaves, so the doorbell and the tablet use microwaves. Bluetooth uses radio waves, so the game controller does. Optical fibres, for cable television and high-speed broadband, carry visible light or infrared.

A full answer includes

I matched the smart doorbell on wi-fi to microwaves.
I matched the Bluetooth game controller to radio waves.
I matched the cable-television fibre to visible light or infrared.
I matched the tablet on the mobile network to microwaves.
I did not match the Bluetooth controller to microwaves.

Also accept

doorbell microwaves; controller radio waves; fibre visible light or infrared; tablet microwaves

Watch for

“The Bluetooth controller uses microwaves, like wi-fi.” — For your course, Bluetooth uses radio waves, which pass through walls but are weakened.
“The cable-television fibre carries microwaves.” — Glass fibres carry visible light or infrared. Microwaves are sent through the air and space.
“The tablet on the mobile network uses radio waves.” — Mobile phone networks use microwaves: they get through some walls and need only a short aerial.
“The doorbell on wi-fi uses visible light or infrared.” — Wi-fi has to reach through walls from the router, so it uses microwaves.

9. CQ9 · Practice

Options as numbered on the pupil sheet: 1 — they cannot pass through walls, so the signal stays private; 2 — they travel faster through the air than waves of other regions; 3 — they get through walls but are weakened, fine for a short link; 4 — they get through some walls, and only need a short aerial.

Model answer. Mobile phones and wireless internet (wi-fi) use microwaves. Microwaves can penetrate some walls, so a phone or a laptop on wi-fi works indoors, and only a short aerial is needed, so it fits inside. Radio waves pass through walls but the signal is weakened on doing so, which suits Bluetooth’s short links between nearby devices. No electromagnetic wave is faster than another in a vacuum, and radio waves do pass through walls.

A full answer includes

I matched mobile phones to “they get through some walls, and only need a short aerial”.
I matched wi-fi to “they get through some walls, and only need a short aerial”.
I matched Bluetooth to “they get through walls but are weakened, fine for a short link”.
I did not choose “travel faster” for any of them.

Also accept

phones and wi-fi: some walls, short aerial; Bluetooth: through walls but weakened, a short link

Watch for

“Mobile phones use microwaves because microwaves travel faster than radio waves.” — All electromagnetic waves travel at the same speed in a vacuum. The reasons are the walls and the short aerial.
“Bluetooth radio waves cannot pass through walls, so the signal stays private.” — They do pass through walls, but the signal is weakened on doing so.
“Bluetooth uses radio waves because they get through some walls and only need a short aerial.” — That is the reason given for microwaves. For Bluetooth, the reason is that radio waves pass through walls but are weakened: enough for a short link.
“Mobile phones use microwaves because they are weakened by walls, which is fine for a short link.” — A phone must work indoors and far from the mast. Microwaves get through some walls and need only a short aerial.

10. CQ10 · Practice

Model answer. (a) Wi-fi uses microwaves. Microwaves can penetrate some walls and floors, so the signal from the router reaches the tablet upstairs, and only a short aerial is needed to send and receive them, so it fits inside the tablet. (b) Bluetooth uses radio waves. They pass through walls, but the signal is weakened on doing so, and it is only strong enough for a short link: once the speaker is outside and across the car park, the signal is too weak, and the music cuts out.

A full answer includes

(a) I wrote that wi-fi uses microwaves.
(a) I wrote that microwaves can pass through some walls and floors.
(a) I wrote that microwaves need only a short aerial.
(b) I wrote that Bluetooth uses radio waves.
(b) I wrote that the walls weaken the signal, so it is too weak once the speaker is outside.
(b) I did not write that radio waves cannot pass through walls.

Also accept

(a) microwaves: some walls and floors; short aerial (b) radio waves: weakened by walls, short link

Watch for

“The music stops because radio waves cannot pass through walls.” — Radio waves do pass through walls, but are weakened; the link only works over a short distance.
“(a) The tablet needs a long aerial to receive microwaves through a floor.” — Microwaves need only a short aerial to send and receive them, which fits inside a thin case.

11. CQ11 · Practice

Model answer. An optical fibre is a long strand of very clear glass. Glass is transparent to visible light and some infrared, so they travel many kilometres inside it, but glass absorbs much of the ultraviolet, which would soon fade away. Ultraviolet does not travel more slowly, and its frequency is higher than that of infrared, not lower. Walls play no part inside a fibre.

A full answer includes

I chose: glass is transparent to some infrared, but it absorbs much ultraviolet.
I did not choose the reason about speed: all electromagnetic waves travel at the same speed in a vacuum.

Also accept

glass is transparent to some infrared, but it absorbs much ultraviolet

Watch for

“Infrared travels faster than ultraviolet, so the data arrives sooner.” — All electromagnetic waves travel at the same speed in a vacuum. The reason is what the glass lets through.
“Ultraviolet has too low a frequency to carry high rates of data.” — Ultraviolet has a higher frequency than infrared. The problem is that glass absorbs much of it.
“Infrared can pass through walls, so the fibre can run through buildings.” — The infrared stays inside the glass of the fibre. What matters is that the glass is transparent to it.

12. CQ24 · Practice

Model answer. The fibre carries infrared (or visible light), not microwaves or an electric current. Glass is transparent to some infrared, so it travels a long way inside the fibre. Short-wavelength infrared can carry high rates of data, so one fibre can bring 400 channels and fast broadband at once. All electromagnetic waves travel at the same speed in a vacuum, so speed is not the reason. Row B.

A full answer includes

I chose row B.
I did not choose row C, which gives speed as the reason for the 400 channels.

Also accept

Row B

Watch for

“Row A.” — Passing through some walls with a short aerial is why phones use microwaves. A fibre carries visible light or infrared, which can carry high rates of data.
“Row C.” — All electromagnetic waves travel at the same speed in a vacuum. The fibre suits 400 channels because infrared can carry high rates of data.
“Row D.” — A fibre carries light or infrared, not an electric current, and glass is an insulator. The infrared travels far because glass is transparent to it.

13. CT1 · Apply

Model answer. (a) Seen from one ground station, the space station crosses the sky in a few minutes, so a dish on the ground could reach it only for those few minutes. A geostationary satellite stays above one point on the equator, so the control room’s fixed dish can point at one all the time. With a few of them spread round the equator, one is always in view of the space station, wherever it is in its orbit, so the calls never stop. (b) Distance up and down = speed × time = 3.0 × 10⁸ m/s × 0.0028 s = 8.4 × 10⁵ m. The height is half of this: 4.2 × 10⁵ m = 420 km. (c) Infrared can carry high rates of data, higher than microwaves, so the video comes down sooner. But thick cloud blocks infrared, as it blocks visible light, while microwaves pass through it: on days with thick cloud over the telescope, the laser link fails.

Checked automatically

(b) Supplement: Height of the space station: 420 km (within ±5 km)
- Common wrong answer 840 : That is the distance up and down. The height is half of it.
- Common wrong answer 0.42 : Check the conversion: 1 km = 1000 m, so 4.2 × 10⁵ m is 420 km.

A full answer includes

(a) I wrote that a dish on the ground could reach the space station for only a few minutes as it crosses the sky.
(a) I wrote that a geostationary satellite stays above one point on the equator, so the control room’s fixed dish can always point at one.
(a) I wrote that with a few satellites round the equator, one is always in view of the space station.
(b) Method: I multiplied 3.0 × 10⁸ m/s by 0.0028 s to get 8.4 × 10⁵ m, up and down.
(b) Method: I halved it for the height alone: 4.2 × 10⁵ m.
(b) Answer: I wrote 420 km.
(c) I wrote that infrared can carry high rates of data.
(c) I wrote that thick cloud blocks infrared, so the link fails on cloudy days.
(b) I did not give 840 km, the distance up and down.

Also accept

(a) the space station crosses a ground dish’s sky in minutes; geostationary satellites stay put for the fixed dish, and one is always in view (b) 420 km (c) infrared: high rates of data; thick cloud blocks it

Watch for

“(a) The calls go through geostationary satellites because they are closer to the space station.” — They are much further away, about 36 000 km up. The point is that they stay put in the sky for the control room’s fixed dish.
“The height is 840 km.” — The 0.0028 s is for the trip up and down. The height is half of the distance.
“(c) The laser link is quicker because infrared travels faster than microwaves.” — All electromagnetic waves travel at the same speed in a vacuum. Infrared can carry higher rates of data: more every second.