Electromagnetic spectrum

Key points

The regions of the spectrum

• Electromagnetic waves are a family of transverse waves (see 3.1). Light is one of them.
• The electromagnetic spectrum is the whole range of electromagnetic waves. It is split into seven main regions.
• In order of increasing frequency:
  • radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays.
• In order of increasing wavelength, the order is reversed:
  • gamma rays → X-rays → ultraviolet → visible light → infrared → microwaves → radio waves.
• So radio waves have the lowest frequency and the longest wavelength. Gamma rays have the highest frequency and the shortest wavelength.
• Visible light is a small part of the spectrum. Red light is next to infrared. Violet light is next to ultraviolet (see 3.2.4).
• All electromagnetic waves travel at the same, very high speed in a vacuum.
• Because the speed is the same, v = fλ shows that a higher frequency means a shorter wavelength.
• The speed of electromagnetic waves in a vacuum is 3.0 × 10⁸ m/s. Extended
• The speed in air is approximately the same. Extended

Uses and dangers

• Radio waves:
  • Radio and television programmes are broadcast with radio waves.
  • In astronomy, radio telescopes detect radio waves given out by stars and galaxies.
  • RFID means radio frequency identification. A reader sends out radio waves. A small tag (for example in a contactless card or on a parcel) sends back a code that identifies it.
• Microwaves:
  • They carry satellite television signals and mobile phone calls.
  • In a microwave oven, the water in food absorbs the microwaves. This increases the thermal energy of the food, so it heats up.
• Infrared:
  • An electric grill gives out infrared, which is absorbed by the surface of the food and cooks it.
  • A television remote controller sends a coded infrared signal over a short distance.
  • An intruder alarm detects the infrared given out by a warm human body.
  • Thermal imaging cameras form a picture from infrared. Warmer objects give out more infrared, so they show up clearly, even in the dark.
  • Optical fibres can carry signals as infrared.
• Visible light: we use it to see (vision), to take photographs (photography) and to light up rooms and streets (illumination).
• Ultraviolet:
  • Security marking: a special ink is invisible in normal light but glows under an ultraviolet lamp. It is used to mark valuable things.
  • Fake bank notes: real notes have marks that glow under ultraviolet. Fake notes do not have the same marks.
  • Sterilising water: ultraviolet kills bacteria and other microorganisms in water.
• X-rays:
  • Medical scanning: X-rays pass through soft tissue but are absorbed by bone. So bones show up clearly on the image.
  • Security scanners at airports use X-rays to see inside bags.
• Gamma rays:
  • Sterilising food and medical equipment: gamma rays kill bacteria. They pass through packaging (see 5.2).
  • Cancer: a gamma-emitting substance in the body helps to detect cancer. Beams of gamma rays aimed at a tumour are used to treat cancer.
• Harmful effects of too much exposure:
  • Microwaves heat the body cells from the inside (internal heating).
  • Infrared causes skin burns.
  • Ultraviolet damages the cells at the surface of the skin and damages the eyes. This can lead to skin cancer and eye conditions.
  • X-rays and gamma rays are ionising. They can cause mutations (changes to the DNA in cells) or damage cells in the body. This can lead to cancer.
• Extra detail: sunscreen and sunglasses reduce exposure to ultraviolet. Staff who take X-ray images stand behind a screen, so they are not exposed many times a day.

Communication

• Communication with artificial satellites is mainly by microwaves.
• A geostationary satellite orbits above the equator once a day, in the same direction as the Earth turns. So it stays above the same point on the Earth, and a dish can point at it all the time.
• Direct broadcast satellite television uses geostationary satellites. So do some satellite phones.
• A low orbit satellite is much closer to the Earth. It moves across the sky. Some satellite phones use low orbit satellites.
• Mobile phones and wireless internet (Wi-Fi) use microwaves. Reasons: microwaves can pass through some walls, and they need only a short aerial to send and receive them. So the aerial fits inside a phone. Extended
• Bluetooth uses radio waves. Radio waves pass through walls, but the signal gets weaker as it does. Bluetooth links devices that are close together, such as a phone and wireless earphones. Extended
• Optical fibres carry visible light or infrared. They are used for cable television and high-speed broadband. Extended
• Two reasons for optical fibres: Extended
  • glass is transparent to visible light and some infrared, so little of the signal is absorbed
  • visible light and short-wavelength infrared can carry data at a high rate (a lot of data each second).
• Extra detail: visible light and infrared have very high frequencies. So the signal can be switched on and off a very large number of times each second.

Analogue and digital signals

• An analogue signal changes continuously. It can have any value within a range. Extended
• A digital signal has only a few fixed values, usually two: on and off, or 1 and 0. It is a series of pulses. Extended
• Sound can be sent as either kind of signal. A microphone turns sound into an analogue electrical signal. This can be sent as it is, or it can be changed into a digital signal of 1s and 0s and sent. Extended
• Every signal gets weaker as it travels. It also picks up noise: unwanted random changes in the signal. So signals must be boosted at intervals along the way. Extended
• Analogue: when the signal is amplified, the noise is amplified too. The noise builds up, and it cannot be removed. Extended
• Digital: a regenerator only has to decide whether each pulse is a 1 or a 0. It then makes a new, clean copy of the pulses. The noise is removed. This is accurate signal regeneration. Extended
• Benefits of digital signalling: Extended
  • increased range: the signal can be regenerated accurately again and again, so it can travel much further without losing its quality
  • increased rate of transmission of data: more data can be sent each second.
• Extra detail: digital data can be compressed, and many digital signals can share one cable or link. This is one reason more data can be sent each second.

Common mistakes

• Students put the regions in the wrong order, or put visible light next to X-rays. / The mark scheme wants the full order: radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma (increasing frequency).
• Students write that gamma rays have the longest wavelength. / The mark scheme wants: gamma rays have the highest frequency and the shortest wavelength.
• Students write that X-rays travel faster than radio waves. / The mark scheme wants: all electromagnetic waves travel at the same speed in a vacuum.
• Students give a vague harm such as “it is dangerous” or “it causes cancer” for every region. / The mark scheme wants the specific effect: microwaves → internal heating; infrared → skin burns; ultraviolet → damage to surface cells and eyes; X-rays and gamma rays → mutation or damage to cells.
• Students write that a digital signal has no noise. / The mark scheme wants: a digital signal does pick up noise, but it can be regenerated accurately, which removes the noise. Extended
• Students write “3 × 10⁸ km/s”. / The mark scheme wants: 3.0 × 10⁸ m/s. Extended

Exam tips

• State the order both ways. Learn one order and reverse it for the other.
• State a use: name the region and the use. If the question asks “why”, add the property that makes it work, for example “X-rays pass through soft tissue but are absorbed by bone”.
• Describe a harmful effect with the exact words for that region. Do not just write “cancer” for every region.
• State the speed with its unit: 3.0 × 10⁸ m/s in a vacuum, about the same in air. Extended
• Calculate with v = fλ and v = 3.0 × 10⁸ m/s. Type powers of ten carefully into your calculator. Extended
• Explain the benefits of digital signals in steps: noise is picked up → a digital signal only has 1s and 0s → it can be regenerated accurately → so the range is greater. Extended
• A typical 1-mark answer: “Satellite television uses microwaves.”
• A typical 2-mark answer to “Why do mobile phones use microwaves?”: “Microwaves can pass through some walls (1). Only a short aerial is needed to send and receive them (1).” Extended
• A typical 3-mark answer to “Explain one benefit of digital signals”: “All signals pick up noise as they travel (1). A digital signal has only two values, so it can be regenerated accurately and the noise is removed (1). So the signal can travel a greater distance (1).” Extended