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P12.2 Light, Infrared, Microwaves, and Radio Waves

GCSE Combined Science · AQA · Chapter P12: Electromagnetic Waves

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: white light, and the uses and dangers of the long-wavelength end of the spectrum. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain what white light is and why a prism or raindrops show the colours in it
  • Describe uses of visible light, infrared radiation, microwaves and radio waves
  • (Higher tier) explain why each type of wave is suitable for its use
  • Explain how a microwave oven heats food and how to test what microwaves pass through
  • Describe the hazards of infrared radiation, microwaves and radio waves

Key words

  • White light: Light that contains all the colours (wavelengths) of the visible spectrum, e.g. sunlight.
  • Infrared (IR): EM radiation with wavelengths just longer than red light. All objects emit it; hotter objects emit more.
  • Microwaves: EM waves with wavelengths from about 1 mm to about 10 cm, between infrared and radio waves.
  • Radio waves: The longest-wavelength EM waves (from about 10 cm up to kilometres), used for broadcasting.
  • Ionosphere: A layer of charged particles high in the atmosphere that reflects some radio waves back to the ground, so they can travel long distances.
  • Thermal imaging: Making a picture from the infrared radiation that objects emit, so warm objects show up in the dark.

White light and visible light

Light from the Sun or a lamp is white light: a mixture of every colour of the visible spectrum. A glass prism splits it up because each wavelength is refracted by a slightly different amount. Violet light (shortest wavelength) bends the most and red light (longest wavelength) the least. A rainbow is the same effect in raindrops.

Uses of visible light:

  • Seeing – our eyes detect only this small part of the spectrum.
  • Optical fibre communication. Pulses of light (or infrared) carry signals along thin glass fibres (see P12.3).
  • Laser surgery – a narrow, intense beam of light can cut or seal tissue, for example to reshape the front of the eye.

Infrared radiation

All objects emit infrared radiation. The hotter an object is, the more infrared it emits each second. When your skin absorbs infrared it warms up – this is the warmth you feel from a fire or a radiator.

  • Electric heaters, grills, toasters and halogen hobs; Why infrared is suitable (Higher tier) Infrared is absorbed by the surface of food or skin and heats it quickly. Halogen hobs emit far more infrared than ordinary hobs, so they heat food faster.
  • Thermal imaging cameras (seeing in the dark, finding people under rubble, spotting heat loss from buildings); Why infrared is suitable (Higher tier) Warm objects emit more infrared than cooler surroundings, so they stand out even when there is no visible light.
  • Medical scanners; Why infrared is suitable (Higher tier) A hot spot on the skin emits extra infrared and can show that the tissue below is unhealthy.
  • TV remote controls; Why infrared is suitable (Higher tier) Short infrared pulses are easy to make and detect electronically, and they do not pass through walls, so they do not affect the TV next door.
  • Optical fibres; Why infrared is suitable (Higher tier) Glass absorbs less infrared than visible light, so signals travel further before they need boosting.

Hazard: Too much infrared can burn the skin: the energy absorbed heats skin cells and can damage or kill them.

Microwaves

Microwaves have shorter wavelengths than radio waves. They are used for:

  • Satellite communication and satellite TV – microwaves pass through the atmosphere to satellites in orbit and back (P12.3).
  • Mobile phone and Wi-Fi signals – they can carry a lot of information.
  • Cooking – microwave ovens heat food quickly.

Microwaves penetrate a few centimetres into food and are absorbed by the water molecules, which gain energy, so the food heats from inside as well as at the surface. The metal walls (and the metal mesh in the door) reflect microwaves and keep them in. Glass, china and plastic contain no water and absorb very little, so a dish only gets warm from the hot food.

Worked example: energy from a microwave oven

A 1000 W microwave oven runs for 1.5 minutes. Calculate the energy it transfers.
t = 1.5 × 60 = 90 s
E = P t = 1000 × 90 = 90 000 J

Answer: 9.0 × 10⁴ J (90 kJ)

Hazard: Microwaves and radio waves penetrate the body and can heat internal tissues if the intensity is high. Ovens are sealed so that almost none escape.

Testing what microwaves pass through. You can test materials with a school microwave transmitter and receiver. With nothing in between, note the receiver reading. Then put a sheet of each material in the gap, at the same position, and note the new reading. A large drop means the material absorbs or reflects the microwaves.

Results: Metals reflect microwaves; water absorbs them strongly; dry paper, cardboard, glass and plastic transmit most of them.

Radio waves

Radio waves have frequencies from about 3 × 10⁵ Hz up to about 3 × 10⁹ Hz, where microwaves begin. They are used for radio and TV broadcasting, two-way radios (e.g. taxi firms and emergency services), and short-range wireless links between a computer and a printer, mouse or headphones that work over about 10 m.

(Higher tier) Radio waves are suitable because they pass easily through the air and through walls, and longer radio waves can reflect off the ionosphere and follow the curve of the Earth, so one transmitter can reach a large area. Signals sent across the ground from a transmitter to homes are called terrestrial signals. Radio waves are produced by oscillating electric currents in aerials (see P12.3).

Worked example: wavelength of a digital radio signal

A digital radio station broadcasts at 220 MHz = 2.2 × 10⁸ Hz. Calculate the wavelength.
λ = v ÷ f = 3.0 × 10⁸ ÷ 2.2 × 10⁸ = 1.36 m

Answer: λ = 1.4 m (2 s.f.)

Summary: uses and hazards

  • Radio waves; Main uses radio and TV broadcasting, two-way radio, short-range wireless links; Hazard very high intensities can heat body tissue
  • Microwaves; Main uses satellite links, mobile phones and Wi-Fi, cooking; Hazard heat internal body tissue
  • Infrared; Main uses heaters, grills, toasters, thermal cameras, remote controls, optical fibres; Hazard skin burns
  • Visible light; Main uses seeing, optical fibres, laser surgery; Hazard very bright light (e.g. lasers) can damage the eye

Common mistakes

  • Thinking only hot objects emit infrared. All objects do – hotter ones simply emit more.
  • Saying microwave ovens “cook from the inside out”. Microwaves are absorbed in the outer few centimetres; the centre heats by conduction.
  • Saying microwave ovens make food radioactive. Microwaves are not ionising and leave no radioactivity.
  • Mixing up which wave a remote control uses (infrared) and which a mobile phone uses (microwaves / short radio waves).
  • Forgetting to change minutes into seconds before using E = P t.

Revision questions

Try these, then check your answers below each one.

Question 1. Why does food get hot in a microwave oven? (Choose from: water molecules in the food absorb the microwaves, the metal walls get hot and heat the food, the microwaves make the food radioactive, the microwaves are reflected by the food.)

Answer: water molecules in the food absorb the microwaves

Working: Microwaves are absorbed by water molecules in the food, transferring energy to its thermal store. Food does not become radioactive.

Question 2. A student puts sheets of four materials, P, Q, R and S, between a microwave transmitter and receiver. The bar chart shows the readings. Which material is most likely to be a metal? (Choose from: P, Q, R, S.)

Answer: S

Working: S lets no microwaves through (reading 0): metals reflect microwaves. R (reading 18) is more likely to be something wet, which absorbs microwaves.

Question 3. A 700 W microwave oven runs for 2.5 minutes. Calculate the energy it transfers.

Answer: 105000 J

Working: t = 2.5 × 60 = 150 s; E = P t = 700 × 150 = 105 000 J.

Related lessons

Continue in the full course

This lesson is section P12.2 (AGCSC4102) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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