Physics · 3 Waves

3.3 Electromagnetic spectrum

Before you start

A wave is described by its frequency (in Hz), wavelength (in m) and speed (in m/s).
The wave equation $v = f λ$ , and how to rearrange it for $f$ or $λ$ .
Speed = distance ÷ time, rearranged to time = distance ÷ speed.
Standard form, and the prefixes kilo (k, $10^{3}$ ), mega (M, $10^{6}$ ) and giga (G, $10^{9}$ ).
That light stays inside an optical fibre by total internal reflection (from 3.2).

Syllabus objectives covered (10)

Know the main regions of the electromagnetic spectrum in order of frequency and in order of wavelength
Know that all electromagnetic waves travel at the same high speed in a vacuum
Describe typical uses of the different regions of the electromagnetic spectrum including: (a) radio waves; radio and television transmissions, astronomy, radio frequency identification (RFID) (b) microwaves; satellite television, mobile phones (cell phones), microwave ovens (c) infrared; electric grills, short range communications such as remote controllers for televisions, intruder alarms, thermal imaging, optical fibres (d) visible light; vision, photography, illumination (e) ultraviolet; security marking, detecting fake bank notes, sterilising water (f) X-rays; medical scanning, security scanners (g) gamma rays; sterilising food and medical equipment, detection of cancer and its treatment
Describe the harmful effects on people of excessive exposure to electromagnetic radiation, including: (a) microwaves; internal heating of body cells (b) infrared; skin burns (c) ultraviolet; damage to surface cells and eyes, leading to skin cancer and eye conditions (d) X-rays and gamma rays; mutation or damage to cells in the body
Know that communication with artificial satellites is mainly by microwaves: (a) some satellite phones use low orbit artificial satellites (b) some satellite phones and direct broadcast satellite television use geostationary satellites
Know that the speed of electromagnetic waves in a vacuum is 3.0 × 108 m / s and is approximately the same in air
Know that many important systems of communications rely on electromagnetic radiation including: (a) mobile phones (cell phones) and wireless internet use microwaves because microwaves can penetrate some walls and only require a short aerial for transmission and reception (b) Bluetooth uses radio waves because radio waves pass through walls but the signal is weakened on doing so (c) optical fibres (visible light or infrared) are used for cable television and high-speed broadband because glass is transparent to visible light and some infrared; visible light and short wavelength infrared can carry high rates of data
Know the difference between a digital and analogue signal
Know that a sound can be transmitted as a digital or analogue signal
Explain the benefits of digital signalling including increased rate of transmission of data and increased range due to accurate signal regeneration

Part 1 of 8

One family of waves, one speed

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays look like seven different things. They are one family: the electromagnetic spectrum. Every member is a transverse wave, and every member can travel through a vacuum.
That second fact is the first difference from sound. Sound needs particles to pass the vibration along, so it cannot cross empty space. Light from the Sun crosses about 150 million kilometres of near-vacuum to reach you, and so does every other electromagnetic wave.
The key fact: all electromagnetic waves travel at the same speed in a vacuum. A gamma ray does not overtake a radio wave. They differ in frequency and wavelength, never in their speed through a vacuum.
For the Extended paper you must know the value of that speed. Nothing on the paper gives it to you, so it has to be recalled:
$c = 3.0 \times 10^{8} m/s$
In air the speed is approximately the same, so you use $3.0 \times 10^{8}$ m/s for a wave travelling through air as well. In denser materials such as glass or water electromagnetic waves slow down, which is what causes refraction. The 'same speed' rule is a vacuum rule, with air as a close enough match.
Because the speed is fixed, the wave equation $v = f λ$ links frequency and wavelength for every region of the spectrum. Rearrange it for whichever one the question wants, and put the frequency in hertz and the wavelength in metres.
$λ = \frac{v}{f} f = \frac{v}{λ}$

Part 2 of 8

The order: frequency one way, wavelength the other

Learn the seven regions in one fixed order, from lowest frequency to highest: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays.
A sentence helps if a list will not stick: Rabbits Mate In Very Unusual eXpensive Gardens.
Now the step that turns one list into two answers. In a vacuum $f \times λ$ is $3.0 \times 10^{8}$ m/s for every region. If the frequency is bigger, the wavelength must be smaller by the same factor to keep the product fixed.
So the same list read the other way is the order of wavelength. Radio waves have the lowest frequency and the longest wavelength. Gamma rays have the highest frequency and the shortest wavelength.
The trap is the question's direction word. 'In order of increasing frequency' starts with radio waves. 'In order of increasing wavelength' starts with gamma rays. Read which quantity, and which direction, before you write a single name.
graph diagram — not drawn yet
A horizontal strip divided into seven labelled regions in the order radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays from left to right, with visible light drawn as a narrow band, an arrow above the strip labelled 'frequency increases' pointing right, and an arrow below labelled 'wavelength increases' pointing left.
One strip, two orders: frequency rises to the right while wavelength rises to the left.
The names of the two regions next to visible light also tell you where they sit. In visible light, red has the longest wavelength and violet the shortest. Infrared lies just beyond the red end, at lower frequency. Ultraviolet lies just beyond the violet end, at higher frequency.

Part 3 of 8

Uses: radio waves to infrared

The syllabus gives you a fixed list of uses for each region. Learn the list. Where you can, tie each use to a reason, because an Explain question asks for the reason and the reason also helps you remember the use.
Radio waves carry radio and television transmissions. They are also used in astronomy: stars and galaxies give out radio waves, and radio telescopes detect them.
Radio frequency identification (RFID) also uses radio waves. A small tag holding a chip and an aerial is read by a nearby reader, with no contact. Tags in shop goods, library books and pet microchips work this way.
Microwaves are used for satellite television, mobile phones (cell phones) and microwave ovens. In an oven, the water in the food absorbs the microwaves, so the food is heated.
Infrared is the radiation that hot objects give out, and that is behind most of its uses. An electric grill cooks food with the infrared from its hot element.
Infrared is also used for short range communications such as television remote controllers. The remote sends a coded pattern of infrared pulses to the television.
Intruder alarms detect the infrared given out by a warm human body. Thermal imaging cameras form a picture from the infrared that objects give out, and hotter objects give out more.
Finally, infrared is one of the two kinds of radiation carried in optical fibres. You meet the reason in the communications section below.

Part 4 of 8

Uses: visible light to gamma rays

Visible light is the only region your eyes detect, so its uses are vision, photography (a camera records the light that reaches it) and illumination (lamps and lighting).
Ultraviolet makes some substances glow with visible light. Security marking uses this: an ink that cannot be seen in ordinary light is written on a valuable object, and it glows under an ultraviolet lamp.
Detecting fake bank notes uses the same effect. Many genuine notes carry marks that glow under ultraviolet light, so a forgery without those marks can be spotted.
Ultraviolet also kills bacteria and other microorganisms, so it is used for sterilising water.
X-rays are used for medical scanning. They pass through soft tissue but are absorbed by bone, so bones show up clearly on the image. Security scanners, for example for luggage at airports, use X-rays to see inside bags without opening them.
Gamma rays kill bacteria, so they are used for sterilising food and medical equipment. Gamma rays penetrate well, so equipment can be sterilised after it has been sealed in its packaging. The food or equipment does not become radioactive.
Gamma rays are also used in the detection of cancer and its treatment. For detection, a substance that gives out gamma rays is put into the body and followed from outside. For treatment, beams of gamma rays are aimed at a tumour to kill the cancer cells.

Part 5 of 8

Harmful effects of too much exposure

The objective says excessive exposure. The harm comes from too much of the radiation. Normal, controlled use is a different matter, so do not write that a single use 'causes cancer'.
Microwaves pass into the body and are absorbed by the water in cells, so the harm is internal heating of body cells. The word to write is internal: the heating is inside the body, not just at the skin.
Infrared is absorbed at the surface of the skin, so the harm is skin burns.
Ultraviolet causes damage to surface cells and eyes, which can lead to skin cancer and eye conditions (such as cataracts). For full credit, give both the damage and what it leads to.
X-rays and gamma rays pass deep into the body, so they can affect cells inside it. The harm is mutation or damage to cells in the body. A mutation is a change in a cell's genetic material, and it can lead to cancer.
Notice the pattern as you move along the spectrum. Microwaves and infrared do their harm by heating. Ultraviolet, X-rays and gamma rays harm by damaging cells directly. Keep the two kinds of harm apart: 'X-rays heat the body' is not the effect the syllabus lists for X-rays, so it does not earn the mark.

Part 6 of 8

Talking to satellites

Communication with artificial satellites is mainly by microwaves. Microwaves pass through the atmosphere, and a dish aerial can send them as a narrow beam aimed at the satellite.
A geostationary satellite orbits above the equator once every 24 hours, in the same direction as the Earth spins. So it stays above the same point on the Earth's surface. A dish on a house can be fixed in one direction and never needs to move.
Direct broadcast satellite television uses geostationary satellites, and so do some satellite phones. A geostationary satellite is far away, about 36 000 km above the Earth's surface, so the signal takes a noticeable fraction of a second to go up and come back down.
A low orbit satellite is much closer to the Earth and goes round in a much shorter time, so it moves across the sky. A single one is overhead for only a short time, so a network of many satellites is needed, and a call passes from one to the next.
Some satellite phones use low orbit satellites. The signal has a much shorter distance to travel, so there is a shorter delay. The second worked example below works this out.
The objective says some satellite phones use each kind. Do not write that all satellite phones use one kind. Both kinds exist.

Part 7 of 8

Choosing a wave for a communication system

For each system the syllabus gives a reason the wave was chosen. A question about these asks for the reason, so learn the reason as a phrase.
Mobile phones and wireless internet use microwaves, for two reasons. First, microwaves can penetrate some walls, so a signal reaches you indoors. Second, they only require a short aerial for transmission and reception, so the aerial fits inside a phone or a router.
Hold on to the word some. Thick or metal walls block microwaves, which is why your signal can drop in some buildings. 'Microwaves pass through walls' with no qualification is not what the syllabus says.
Bluetooth uses radio waves. Radio waves pass through walls, but the signal is weakened on doing so. Both halves of that sentence matter. The weakening is one reason Bluetooth works best over short distances, such as headphones in the same room as the phone.
Optical fibres carry visible light or infrared and are used for cable television and high-speed broadband. The light stays inside the fibre by total internal reflection (from the light topic, 3.2).
Two reasons explain why these waves are used in the fibres. First, glass is transparent to visible light and some infrared, so the waves travel along the glass without being absorbed. Second, visible light and short wavelength infrared can carry high rates of data, which is what high-speed broadband needs.

Part 8 of 8

Analogue and digital signals

A signal is a wave or a current that has been varied to carry information. The difference between analogue and digital is about the shape of that variation, not about which wave carries it.
An analogue signal varies continuously and can take any value within a range. A digital signal can take only a few fixed values, usually two, written as 1 and 0 (on and off). It jumps between them.
graph diagram — not drawn yet
Two graphs of voltage against time, one above the other: the upper labelled 'analogue', a smooth wave whose value changes continuously; the lower labelled 'digital', a pulse train that jumps with vertical steps between only two levels marked 1 and 0.
The analogue trace can sit at any height; the digital trace is only ever at one of two levels.
A sound can be transmitted as either kind. A microphone turns sound into an analogue electrical signal, whose voltage varies continuously in step with the sound wave. Sent like that, as FM radio sends it, it is an analogue transmission.
To send the same sound digitally, the analogue signal is measured many times every second. Each measurement is coded as a binary number, a string of 1s and 0s, and the 1s and 0s are sent. Digital radio and mobile phone calls send sound this way.
Now the benefits. Every signal gets weaker as it travels and picks up noise: random, unwanted changes added on top of it. To travel far, the signal must be boosted at points along the way.
An analogue signal is boosted by an amplifier. The amplifier cannot tell the signal from the noise, so it amplifies the noise as well. The noise builds up at every stage, and the sound gets worse the further it goes.
A digital signal is regenerated. The regenerator only has to decide, for each pulse, whether it is a 1 or a 0, and it then sends out a fresh, clean pulse. Unless the noise is big enough to make a 1 look like a 0, the noise is removed completely.
graph diagram — not drawn yet
Three voltage-time traces stacked vertically: an original clean two-level digital pulse train; the same pulse train after travelling along a cable, with smaller pulses and jagged noise added; and the regenerated pulse train, identical to the original.
Weakened and noisy pulses can still be read as 1s and 0s, so the regenerator restores the original exactly.
That is the meaning of accurate signal regeneration, and it gives the first benefit: increased range. Because a digital signal can be restored exactly again and again, it can be sent much further than an analogue one.
The second benefit in the syllabus is an increased rate of transmission of data: digital signalling sends more data every second. When you explain the benefits, state both, and for range give the reason, which is accurate regeneration that removes the noise.

Worked examples

Example 1. A radio station broadcasts at a frequency of 100 MHz. A mobile phone sends microwaves of wavelength 0.15 m. (a) Calculate the wavelength of the radio waves in air. (b) Calculate the frequency of the microwaves. (c) State which of the two waves has the higher frequency, and check that this agrees with the order of the spectrum.

Show the working

Both waves travel at the speed of electromagnetic waves, and in air that is approximately $3.0 \times 10^{8}$ m/s. The question does not give this value. You recall it.
(a) Convert the frequency to hertz first. Mega means $10^{6}$ , so $100$ MHz $= 100 \times 10^{6}$ Hz $= 1.0 \times 10^{8}$ Hz.
Rearrange $v = f λ$ for wavelength by dividing both sides by $f$ , then substitute.
$λ = \frac{v}{f} = \frac{3.0 \times 10^{8}}{1.0 \times 10^{8}} = 3.0 m$
(b) This time the wavelength is given and the frequency is wanted, so divide both sides of $v = f λ$ by $λ$ .
$f = \frac{v}{λ} = \frac{3.0 \times 10^{8}}{0.15} = 2.0 \times 10^{9} Hz$
(c) $2.0 \times 10^{9}$ Hz is bigger than $1.0 \times 10^{8}$ Hz, so the microwaves have the higher frequency. That agrees with the order: microwaves come after radio waves in the list of increasing frequency. The microwaves also have the shorter wavelength, $0.15$ m against $3.0$ m, as they must when the speed is the same.

Answer: (a) 3.0 m (b) $2.0 \times 10^{9}$ Hz (2.0 GHz) (c) The microwaves have the higher frequency, which agrees with microwaves lying above radio waves in frequency.

Example 2. A geostationary satellite is $3.6 \times 10^{7}$ m above the Earth's surface. A low orbit satellite is $1.2 \times 10^{6}$ m above the surface. For each satellite, calculate the time for a microwave signal to travel from a ground station up to the satellite and straight back down to the same place. Use these values to explain why a phone call through a low orbit satellite has a shorter delay.

Show the working

The signal is a microwave, so it travels at $3.0 \times 10^{8}$ m/s. Treat the speed as the same all the way, because it is approximately the same in air as in a vacuum.
The signal goes up and comes back down, so the distance is twice the height. Forgetting the return journey halves the answer.
Use speed = distance ÷ time, rearranged to time = distance ÷ speed. For the geostationary satellite:
$t = \frac{2 \times 3.6 \times 10^{7}}{3.0 \times 10^{8}} = 0.24 s$
For the low orbit satellite:
$t = \frac{2 \times 1.2 \times 10^{6}}{3.0 \times 10^{8}} = 0.008 s$
Both signals travel at the same speed, so the time depends only on the distance. The low orbit satellite is 30 times closer, so the signal takes one thirtieth of the time, and the delay is shorter.

Answer: Geostationary: 0.24 s. Low orbit: 0.008 s ( $8.0 \times 10^{- 3}$ s). The signals travel at the same speed, but the low orbit satellite is much closer, so the signal has a much shorter distance to travel and the delay is shorter.

Example 3. A piece of music is sent along a long cable to a distant town. It can be sent as an analogue signal or converted to a digital signal first. Both signals pick up noise along the way. Explain why the digital signal arrives with the music unchanged but the analogue signal does not.

Show the working

Start with what happens to any signal on a long cable: it weakens and picks up noise, so it has to be boosted at points along the way.
Say what happens to the analogue signal. An amplifier boosts it, but the amplifier cannot separate the signal from the noise, so it amplifies the noise too. The noise builds up at each stage.
Say what happens to the digital signal. It has only two values, 1 and 0. A regenerator only needs to decide which value each pulse has, and then sends out a fresh pulse at the correct level.
Finish with the result. The noise is removed at every regeneration, so the digital signal is restored accurately and the music arrives unchanged. Accurate regeneration is also why a digital signal can be sent further.

Answer: Both signals pick up noise. The analogue signal is amplified and the noise is amplified with it, so the noise builds up. The digital signal has only two levels, so a regenerator can recognise each 1 and 0 and send out clean pulses again. The noise is removed, the signal is regenerated accurately, and the music arrives unchanged.

Where people go wrong

Mistake: Saying gamma rays travel faster than radio waves because they have a higher frequency.

Instead: All electromagnetic waves travel at the same speed in a vacuum, $3.0 \times 10^{8}$ m/s. A higher frequency means a shorter wavelength, not a higher speed.

In $v = f λ$ the speed is fixed for the whole spectrum, so a change in $f$ is balanced by an opposite change in $λ$ .

Mistake: Writing the order of increasing wavelength in the same order as the order of increasing frequency.

Instead: The two orders are opposite. Increasing frequency starts with radio waves. Increasing wavelength starts with gamma rays.

With a fixed speed, the region with the highest frequency must have the shortest wavelength.

Mistake: Calling radio waves a kind of sound.

Instead: Radio waves are electromagnetic waves and can cross a vacuum. Sound is a different kind of wave that needs a material. A radio wave can carry a signal that represents a sound.

The radio converts the signal back into sound only at the end. The wave that crosses the gap is electromagnetic.

Mistake: Saying a digital signal does not pick up noise.

Instead: Both kinds pick up noise. The difference is that a digital signal can be regenerated so that the noise is removed, but an analogue amplifier amplifies the noise with the signal.

The benefit of digital is the accurate regeneration of the signal. It is not protected from noise.

Mistake: Using the frequency in MHz or GHz directly in $v = f λ$ .

Instead: Convert to hertz first: M (mega) is $10^{6}$ and G (giga) is $10^{9}$ , so 2.4 GHz is $2.4 \times 10^{9}$ Hz.

The speed is in m/s, so the equation gives a wavelength in metres only when the frequency is in hertz.

Check yourself

Put these regions in order of increasing wavelength: X-rays, microwaves, visible light, gamma rays, radio waves.
Answer
Gamma rays, X-rays, visible light, microwaves, radio waves.
Ultraviolet has a higher frequency than infrared. Does ultraviolet travel faster than infrared in a vacuum? What is different about it instead?
Answer
No. All electromagnetic waves travel at the same speed in a vacuum. Ultraviolet has a shorter wavelength than infrared.
A microwave oven produces microwaves of frequency 2.45 GHz. Calculate their wavelength.
Answer
$λ = v / f = 3.0 \times 10^{8} / 2.45 \times 10^{9} = 0.12$ m (0.122 m).
The Sun is $1.5 \times 10^{11}$ m from the Earth. Calculate the time for light to travel from the Sun to the Earth.
Answer
$t = d / v = 1.5 \times 10^{11} / 3.0 \times 10^{8} = 500$ s (about 8.3 minutes).
Name the region of the spectrum used for each of these: thermal imaging; detecting fake bank notes; RFID tags; sterilising food; airport luggage scanners.
Answer
Infrared; ultraviolet; radio waves; gamma rays; X-rays.
Describe the harmful effect of excessive exposure to (a) infrared, (b) ultraviolet, (c) microwaves.
Answer
(a) Skin burns. (b) Damage to surface cells and eyes, which can lead to skin cancer and eye conditions. (c) Internal heating of body cells.
(a) Which region does Bluetooth use, and what happens to its signal when it passes through a wall? (b) Give two reasons why optical fibres carry visible light or infrared. (c) Which kind of satellite does direct broadcast satellite television use?
Answer
(a) Radio waves; they pass through the wall but the signal is weakened. (b) Glass is transparent to visible light and some infrared, and visible light and short wavelength infrared can carry high rates of data. (c) Geostationary satellites.
State the difference between an analogue and a digital signal, say whether a sound can be sent as either, and give two benefits of digital signalling.
Answer
An analogue signal varies continuously and can take any value. A digital signal takes only a few values, usually 1 and 0. A sound can be sent as either. Benefits: an increased rate of transmission of data, and an increased range because the signal can be regenerated accurately.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. Some regions of the electromagnetic spectrum are listed in order of increasing frequency: radio waves, P, infrared, visible light, Q, X-rays, R.

(a) Identify the regions P, Q and R. 2 marks No calculator

Mark scheme

B2 P microwaves, Q ultraviolet, R gamma rays; all three correct for 2 marks, any two correct for 1 mark

(b) State how the speed of X-rays in a vacuum compares with the speed of radio waves in a vacuum. 1 mark No calculator

Mark scheme

B1 they are the same (speed), owtte

(c) A radio transmitter sends out waves of frequency $2.0 \times 10^{5}$ Hz. Calculate the wavelength of these waves in air. 3 marks

Mark scheme

C1 $v = f λ$ in any form, or $λ = v / f$
C1 $λ = 3.0 \times 10^{8} / 2.0 \times 10^{5}$
A1 1500 m (accept $1.5 \times 10^{3}$ m)

Question 2. A hospital uses several kinds of electromagnetic radiation. Gamma rays are used to sterilise surgical instruments, and X-rays are used to produce images of patients' bones.

(a) Give one other use of gamma rays in a hospital. 1 mark No calculator

Mark scheme

B1 any one from: detecting cancer / treating cancer (radiotherapy); ignore sterilising

(b) Describe the harmful effects on a person of excessive exposure to X-rays. 2 marks No calculator

Mark scheme

B1 damage to cells (in the body)
B1 mutation (of cells) / change to the genetic material (DNA) of cells, owtte; accept (can lead to) cancer

(c) A microwave oven in the staff room is designed so that it cannot operate while its door is open. Explain why. 2 marks No calculator

Mark scheme

B1 microwaves would escape from the oven and reach people nearby, owtte
B1 (excessive exposure to) microwaves causes internal heating of body cells; insufficient: 'microwaves are dangerous'

Question 3. A house receives television from a geostationary satellite. The family also use mobile phones and wireless internet inside the house.

(a) State the region of the electromagnetic spectrum used to communicate with the satellite. 1 mark No calculator

Mark scheme

B1 microwaves

(b) Explain why the satellite dish on the house can stay pointing in one fixed direction. 2 marks No calculator

Mark scheme

B1 the satellite orbits once every 24 hours in the same direction as the Earth spins, owtte
B1 so it stays above the same point on the Earth's surface

(c) Give two reasons why mobile phones and wireless internet use microwaves. 2 marks No calculator

Mark scheme

B1 microwaves can penetrate (some) walls
B1 only a short aerial is needed for transmission and reception

Question 4. A singer's voice is picked up by a microphone, which produces an electrical signal that varies continuously. The sound is to be sent to a radio studio in another city.

(a) Describe the difference between an analogue signal and a digital signal. 2 marks No calculator

Mark scheme

B1 an analogue signal varies continuously / can take any value (within a range)
B1 a digital signal has only a few (fixed) values, e.g. two values, 1 and 0 / on and off

(b) The signal is converted to a digital signal before it is sent. Explain the benefits of sending the sound as a digital signal. 3 marks No calculator

Mark scheme

B1 increased rate of transmission of data
B1 the (weakened, noisy) digital signal can be regenerated accurately / the noise is removed
B1 so the signal has an increased range / can be sent further without loss of quality

How this is examined

Matching each region of the spectrum to its uses, and naming the property behind the use: a one- or two-mark Give or State: name the region for a stated use, or give a use for a stated region; an Explain asks why that region suits the job The region named exactly as in the syllabus list (for example 'gamma rays' for sterilising medical equipment). In an Explain, a property tied to the use: 'X-rays pass through soft tissue but are absorbed by bone'.
What too much of each radiation does to the body: a two-mark Describe or Explain, can be set in a situation such as a microwave oven door or a sunbed The syllabus effect for the right region: 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.
Using the recalled speed $3.0 \times 10^{8}$ m/s in $v = f λ$ : a three-mark Calculate: a frequency in kHz, MHz or GHz and a wavelength wanted, or the reverse The equation in any correct form (C1), the substitution with $3.0 \times 10^{8}$ m/s and the frequency in hertz (C1), and the answer with its unit (A1). A missing unit loses the final mark.
Why a digital signal can be sent further than an analogue one: a two- or three-mark Explain about a signal crossing a long distance The noise is picked up on the way; a digital signal is regenerated accurately because only the 1s and 0s need to be recognised, so the noise is removed; this gives increased range. Add 'increased rate of transmission of data' as the second benefit.
Why each communication system uses the wave it does: a two-mark Give or Explain about phones, wireless internet, Bluetooth, optical fibres or satellites The syllabus reasons as phrases: microwaves 'can penetrate some walls' and 'only require a short aerial'; Bluetooth radio waves 'pass through walls but the signal is weakened'; glass 'is transparent to visible light and some infrared'; a geostationary satellite 'stays above the same point on the Earth's surface'.
Both orders of the spectrum: a one- or two-mark Identify or State: missing regions in a list, or the region with the longest or shortest wavelength The seven regions in the right order for the quantity asked: radio waves to gamma rays for increasing frequency, gamma rays to radio waves for increasing wavelength.

Words to know

Electromagnetic spectrum: The family of transverse waves that all travel at $3.0 \times 10^{8}$ m/s in a vacuum: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays, in order of increasing frequency.
Geostationary satellite: A satellite that orbits once every 24 hours in the same direction as the Earth spins, so it stays above the same point on the Earth's surface.
Low orbit satellite: A satellite close to the Earth that moves across the sky, so a network of them is needed for continuous communication.
RFID: Radio frequency identification: reading a small tag from a distance using radio waves.
Analogue signal: A signal that varies continuously and can take any value within a range.
Digital signal: A signal that can take only a few fixed values, usually two (1 and 0).
Noise: Random, unwanted changes added to a signal as it travels.
Regeneration: Restoring a weakened, noisy digital signal to clean pulses by recognising each 1 and 0 and sending it out again.
Mutation: A change in the genetic material of a cell, which can lead to cancer.
Sterilising: Killing bacteria and other microorganisms on or in something.

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