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13.2 Light, Infrared, Microwaves and Radio Waves

GCSE Physics · AQA · Chapter 13: Electromagnetic Waves

A free GCSE Physics lesson for AQA (8463) 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 how a prism shows 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.
  • Pixel: One of the millions of tiny light-sensitive cells in a camera sensor. Each one records one dot of the image.
  • 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 and photography. In a digital camera or phone, a lens focuses light on to a sensor made of millions of pixels. Each pixel records the brightness and colour of one dot of the image, which is stored electronically (an older film camera focuses the light on to light-sensitive film instead).
  • Optical fibre communication. Pulses of light (or infrared) carry signals along thin glass fibres (see 13.3).

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 (13.3).
  • 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 900 W microwave oven runs for 2.0 minutes. Calculate the energy it transfers.
t = 2.0 × 60 = 120 s
E = P t = 900 × 120 = 108 000 J

Answer: 1.1 × 10⁵ J (108 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.

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 long wavelengths travel long distances and spread out (diffract) around hills and buildings, so one transmitter can reach a large area. They are produced by oscillating electric currents in aerials (see 13.3).

Worked example: wavelength of a wireless headphone signal

Wireless headphones use a frequency of 2.4 GHz. Calculate the wavelength.
λ = v ÷ f = 3.0 × 10⁸ ÷ 2.4 × 10⁹ = 0.125 m

Answer: λ = 0.13 m (12.5 cm)

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. To check for reflection, place a metal sheet at an angle and move the receiver round to find the reflected beam.

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

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, photography, optical fibres; 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. Which type of electromagnetic wave does a TV remote control use? (Choose from: Infrared, Microwaves, Ultraviolet, Radio waves.)

Answer: Infrared

Working: Infrared. Pulses of infrared are easy to make and detect, and they stay in the room.

Question 2. True or false? The hotter an object is, the more infrared radiation it emits.

Answer: True

Working: True. All objects emit infrared, and hotter objects emit more of it each second.

Question 3. A microwave oven uses microwaves with a frequency of 2450 MHz. Their speed is 3.0 × 10⁸ m/s. Calculate the wavelength in centimetres, to 3 significant figures.

Answer: 12.2 cm

Working: f = 2.45 × 10⁹ Hz. λ = 3.0 × 10⁸ ÷ 2.45 × 10⁹ = 0.122 m = 12.2 cm.

Related lessons

Continue in the full course

This lesson is section 13.2 (AGCPH1302) of our free GCSE Physics 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 Physics course.

 

 

 

 

 

 

 

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