Cambridge IGCSE0972

Electromagnetic spectrum

Physics 0972 Chapter Notes

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Electromagnetic spectrum
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In this chapter

  1. 1Properties of Electromagnetic Waves
  2. 2The Electromagnetic Spectrum
  3. 3Uses: Radio, Microwaves & Infrared
  4. 4Uses & Dangers: Visible, UV, X-rays & Gamma

1. Properties of Electromagnetic Waves

The electromagnetic (EM) spectrum is a family of waves that share several key properties. They are all transverse waves, meaning their oscillations are at right angles to their direction of travel. Unlike sound waves, they do not need a medium to travel and can pass through a vacuum, like the space between the Sun and Earth. All EM waves travel at the same incredibly high speed in a vacuum: 300,000,000 metres per second (3 x 10^8 m/s), also known as the speed of light. They are created by oscillating electric charges and transfer energy from a source to an absorber.

c = f × λ

Key term

Transverse Wave: A wave in which the oscillations are perpendicular (at 90°) to the direction of energy transfer.

Examiner insight

Examiners frequently ask for properties common to all electromagnetic waves; memorising that they are transverse, travel at the speed of light in a vacuum, and transfer energy will secure easy marks.

Common pitfall

Confusing electromagnetic waves with mechanical waves (like sound), which require a medium to travel.

Fun fact

If you could travel at the speed of light, you could circle the Earth's equator approximately 7.5 times in just one second.

Worked example 13 marks

A Wi-Fi router emits waves with a frequency of 2,400 MHz. Calculate their wavelength. The speed of electromagnetic waves in a vacuum is 3.0 x 10^8 m/s.

  1. 1
    1. State the formula: speed = frequency × wavelength (c = fλ).
  2. 2
    1. Rearrange the formula to find wavelength: λ = c / f.
  3. 3
    1. Convert the frequency from MHz to Hz: 2,400 MHz = 2,400 × 1,000,000 Hz = 2.4 × 10^9 Hz.
  4. 4
    1. Substitute the values into the formula: λ = (3.0 × 10^8 m/s) / (2.4 × 10^9 Hz).
  5. 5
    1. Calculate the result: λ = 0.125 m.

Recap

  • All EM waves are transverse.
  • All EM waves travel at the speed of light (3 x 10^8 m/s) in a vacuum.
  • EM waves can travel through a vacuum and do not require a medium.
  • EM waves transfer energy from a source to an absorber.
  • The wave speed equation is c = f × λ, where 'c' is the speed of light.

Quick check

  1. State two properties that are the same for both radio waves and X-rays.2 marks

2. The Electromagnetic Spectrum

The electromagnetic spectrum is the complete range of EM waves, organised by their wavelength and frequency. It is a continuous spectrum, but we divide it into seven main regions for convenience. In order of increasing frequency (and decreasing wavelength), they are: Radio waves, Microwaves, Infrared, Visible light, Ultraviolet, X-rays, and Gamma rays. A helpful mnemonic to remember this order is 'Raging Martians Invaded Venus Using X-ray Guns'. As you move from radio waves to gamma rays, the frequency and energy of the waves increase, while their wavelength decreases.

Key term

Electromagnetic Spectrum: The continuous range of electromagnetic waves, from radio waves to gamma rays, arranged in order of frequency or wavelength.

Examiner insight

Marks are consistently awarded for correctly listing the parts of the EM spectrum in order. Questions often show a diagram of the spectrum with a missing section for you to identify.

Common pitfall

Mixing up the order of Infrared and Ultraviolet. Remember that Infrared (IR) is 'below red' and Ultraviolet (UV) is 'beyond violet' in the visible spectrum.

Fun fact

Visible light, the only part of the spectrum our eyes can detect, makes up less than 0.0035% of the entire electromagnetic spectrum.

Worked example 11 mark

The diagram shows the electromagnetic spectrum with one part missing. | Radio waves | Microwaves | P | Visible light | Ultraviolet | X-rays | Gamma rays |. Name the radiation found in the region labelled P.

  1. 1
    1. Recall the order of the electromagnetic spectrum: Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma ray.
  2. 2
    1. Identify the waves on either side of P. P is between Microwaves and Visible light.
  3. 3
    1. The wave that sits between microwaves and visible light is Infrared.
  4. 4
    1. Therefore, P is Infrared radiation.

Worked example 22 marks

Place the following electromagnetic waves in order of increasing wavelength: Gamma rays, Infrared, Radio waves, Ultraviolet.

  1. 1
    1. Recall the relationship between wavelength and frequency: as frequency increases, wavelength decreases. Therefore, increasing wavelength means decreasing frequency.
  2. 2
    1. Recall the order of the spectrum by frequency (lowest to highest): Radio, Infrared, Ultraviolet, Gamma.
  3. 3
    1. Reverse this order to get the order of increasing wavelength.
  4. 4
    1. The correct order is: Gamma rays, Ultraviolet, Infrared, Radio waves.

Recap

  • The EM spectrum is a continuous range of waves.
  • The order of increasing frequency is: Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma ray.
  • The order of increasing wavelength is the reverse: Gamma ray, X-ray, Ultraviolet, Visible, Infrared, Microwave, Radio.
  • Frequency and wavelength are inversely proportional: high frequency means short wavelength.
  • The energy of an EM wave increases with its frequency.

Quick check

  1. Which type of electromagnetic wave has a wavelength just shorter than that of red light?1 mark
  2. Which has more energy: a microwave photon or an X-ray photon?1 mark

3. Uses: Radio, Microwaves & Infrared

Different parts of the EM spectrum are suited for different jobs. Radio waves have long wavelengths, allowing them to travel long distances and diffract (bend) around obstacles, making them ideal for broadcasting radio (AM, FM) and television signals. Microwaves are used for satellite communications and mobile phones because they can pass through the Earth's atmosphere easily. Their heating effect is used in microwave ovens to cook food by making water molecules vibrate. Infrared (IR) radiation is what we feel as heat. It's used in heaters, grills, thermal imaging cameras (to see in the dark), and in remote controls to send signals.

Key term

Diffraction: The spreading out of waves as they pass through a gap or around an obstacle.

Examiner insight

When asked for a use, be specific. 'Communication' is too vague; 'Satellite communication' or 'TV remote control' are much better answers.

Common pitfall

Thinking that microwave ovens are radioactive. They use non-ionising radiation to heat water molecules; the food does not become radioactive.

Fun fact

The cosmic microwave background radiation, a faint glow of microwaves filling the universe, is leftover energy from the Big Bang.

Worked example 12 marks

Explain why microwaves are used for communications between satellites and the Earth.

  1. 1
    1. Microwaves can pass through the Earth's atmosphere without being significantly reflected or absorbed.
  2. 2
    1. They also have a high frequency, which allows them to carry a large amount of information (high bandwidth).
  3. 3
    1. This allows a clear, high-data signal to travel from the satellite in space to the receiving dish on the ground.

Worked example 22 marks

State two uses of infrared radiation.

  1. 1
    1. Use 1: In television or stereo remote controls to transmit signals.
  2. 2
    1. Use 2: In thermal imaging or 'night vision' cameras to detect heat signatures.

Recap

  • Radio waves are used for broadcasting due to their long wavelength and ability to diffract.
  • Microwaves are used for satellite communication, mobile phones, and heating food.
  • Infrared is used for heating, thermal imaging, and remote controls.
  • These three types of radiation are generally considered non-ionising and are less harmful than high-frequency waves.

Quick check

  1. What property of radio waves makes them suitable for broadcasting over long distances and around hills?1 mark
  2. How does a microwave oven heat food?1 mark

4. Uses & Dangers: Visible, UV, X-rays & Gamma

Visible light is the part of the spectrum we use for sight and in optical fibres for high-speed data communication. Ultraviolet (UV) light from the Sun causes tanning, but overexposure is dangerous and can lead to skin cancer and eye damage. Its high energy is also useful for sterilising water and as a security feature, as it causes some inks to fluoresce (glow). X-rays are highly penetrating and are used in hospitals to image bones and in airport security to check luggage. Gamma rays are even more penetrating and are emitted by radioactive materials. They are used to sterilise medical equipment and to kill cancerous cells in a process called radiotherapy. Both X-rays and Gamma rays are ionising, meaning they have enough energy to damage living cells, which is why exposure must be limited.

Key term

Ionising Radiation: Radiation with enough energy to knock electrons out of atoms, which can damage living cells and cause cancer.

Examiner insight

For high-energy waves like UV, X-rays, and gamma rays, examiners expect you to know both a use and a corresponding danger or safety precaution.

Common pitfall

Confusing the uses of X-rays and gamma rays. While both are used in medicine, X-rays are primarily for imaging bones, while gamma rays are more commonly used for sterilisation and cancer treatment (radiotherapy).

Fun fact

Gamma-ray bursts are the most powerful explosions in the universe. A single burst can release more energy in 10 seconds than our Sun will in its entire 10-billion-year lifetime.

Worked example 13 marks

a) State one use of ultraviolet (UV) radiation.b) State one danger of excessive exposure to UV radiation.c) Describe a precaution you could take to minimise this danger.

  1. 1

    a) Use: Sterilising drinking water or security marking of banknotes.

  2. 2

    b) Danger: It can cause skin cancer or damage to surface eye cells (cataracts).

  3. 3

    c) Precaution: Wear high-factor sunscreen and/or UV-protective sunglasses.

Worked example 23 marks

Explain why radiographers who operate X-ray machines take precautions such as leaving the room during an exposure.

  1. 1
    1. X-rays are a form of ionising radiation.
  2. 2
    1. Ionising radiation can damage living cells and increase the long-term risk of cancer.
  3. 3
    1. While the risk from a single exposure for a patient is very small, a radiographer performs many X-rays each day.
  4. 4
    1. Leaving the room prevents this repeated, cumulative exposure, ensuring their dose remains at a safe low level.

Recap

  • Visible light is used for sight and in optical fibre communications.
  • Ultraviolet (UV) is used for sterilisation and security, but can cause skin cancer.
  • X-rays are used for medical imaging and are ionising.
  • Gamma rays are used for radiotherapy and sterilisation and are highly ionising.
  • Safety precautions like shielding or increasing distance are essential when using ionising radiation (UV, X-rays, Gamma rays).

Quick check

  1. Name two types of electromagnetic radiation that are ionising.2 marks
  2. Why are gamma rays, and not microwaves, used to sterilise medical instruments?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. List the seven types of electromagnetic waves in order, starting with the one with the longest wavelength.2 marks
  2. State three properties that are common to all types of electromagnetic waves.3 marks
  3. A television remote control uses infrared radiation to send signals to the television. Explain why infrared is a suitable choice for this purpose and not radio waves.2 marks
  4. An FM radio station broadcasts on a frequency of 102.5 MHz. Calculate the wavelength of the radio waves. (The speed of electromagnetic waves is 3.0 × 10^8 m/s).3 marks
  5. The table shows some types of EM radiation and their uses. Complete the table by filling in the gaps (i), (ii), (iii) and (iv).4 marks
  6. Explain the difference between an ionising and a non-ionising electromagnetic wave. Give one example of each.3 marks
  7. Overexposure to ultraviolet (UV) radiation can be harmful. Describe one harmful effect and one practical application of UV radiation.3 marks
  8. Both gamma rays and X-rays can be dangerous to humans, but both are also used in hospitals. a) State one medical use for X-rays. b) State one medical use for gamma rays. c) Explain why exposure for medical staff to these radiations is kept to a minimum.4 marks
  9. A student claims that since gamma rays have the shortest wavelength, they must travel the slowest through a vacuum. Explain why this student is incorrect.2 marks
  10. A communications satellite in orbit has a camera that uses visible light to take photographs of weather patterns. The wavelength of green light it detects is 5.5 x 10^-7 m. a) Explain why microwaves, not visible light, are used to transmit data from the satellite to Earth. b) Calculate the frequency of this green light.5 marks

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