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2.3 More About Infrared Radiation

GCSE Physics · AQA · Chapter 2: Energy Transfer by Heating

A free GCSE Physics lesson for AQA (8463) from OBA School: good and poor absorbers and emitters, uses of surfaces, and how radiation affects the temperature of the Earth (physics only, Higher tier). 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

  • Describe which surfaces are the best and worst absorbers and emitters of infrared radiation
  • Describe how to compare the infrared emitted by different surfaces
  • (Higher tier) explain what happens to the temperature of an object that absorbs radiation faster than it emits it, and the reverse
  • (Higher tier) explain how the temperature of the Earth depends on the balance between absorbed and emitted radiation
  • Explain everyday uses such as emergency blankets, vacuum flasks and solar heating panels

Key words

  • Matt: Dull, not shiny. A matt surface scatters light rather than reflecting it like a mirror.
  • Good emitter: A surface that gives out a lot of infrared each second at a given temperature. Matt black is best.
  • Good absorber: A surface that takes in most of the infrared that falls on it. A good absorber is also a good emitter.
  • Reflector: A surface that sends most of the radiation back. Shiny, silvery surfaces are the best reflectors and the worst absorbers and emitters.
  • Greenhouse gas: A gas such as carbon dioxide, methane or water vapour that absorbs longer-wavelength infrared emitted by the Earth.
  • Steady temperature: The temperature an object settles at when it emits radiation at the same rate as it absorbs it.

Absorbing and emitting: what happens to the temperature (Higher tier)

Every object absorbs and emits infrared, whatever its temperature. Comparing the two rates tells you what happens next:

  • absorbs faster than it emits → temperature increases
  • emits faster than it absorbs → temperature decreases
  • absorbs and emits at the same rate → temperature stays constant.

A hotter object emits more radiation per second. So as an object in sunlight warms up, its emission rises until it equals the absorption. The black can absorbs more, so it must reach a higher temperature before the rates balance.

Which surfaces absorb and emit best?

  • matt black; Absorbing infrared best; Emitting infrared best; Reflecting infrared worst
  • glossy (shiny) black; Absorbing infrared good; Emitting infrared good; Reflecting infrared poor
  • light, matt; Absorbing infrared poor; Emitting infrared poor; Reflecting infrared good
  • shiny silver; Absorbing infrared worst; Emitting infrared worst; Reflecting infrared best

You can compare emitters with a hollow metal cube whose four faces have different surfaces. Fill it with hot water so every face is at the same temperature, then point an infrared detector at each face from the same distance.

Worked example: detector readings

Readings at 15 cm: matt black 9.4; glossy black 7.8; matt silver 3.1; shiny silver 1.2 (arbitrary units).
How many times more does matt black emit than shiny silver? 9.4 ÷ 1.2 = 7.8

Answer: about 8 times as much

Fair test: Control variables for this test: same temperature of every face (one cube of water), same distance from the detector, same detector, same size of face, and readings taken quickly so the water does not cool.

Using surfaces to control energy transfer

  • Emergency blankets are thin plastic with a shiny metal coating. The shiny surface is a poor emitter, and it reflects infrared from the body back towards it, so the person loses energy more slowly.
  • Vacuum flasks have silvered walls (poor emitters and good reflectors, so little energy is transferred by radiation) and a vacuum between the walls (no particles, so no conduction or convection across it).
  • Solar heating panels have matt black absorbing plates under glass so they absorb as much of the Sun’s radiation as possible.
  • Buildings in hot countries are often painted white to reflect sunlight and stay cooler.

Exam tip: Always link the surface to the transfer: “shiny, so it is a poor emitter of infrared, so the rate of energy transfer is lower.”

Radiation and the temperature of the Earth (Higher tier)

The Earth’s average temperature depends on the balance between:

  • radiation from the Sun that is reflected back into space (by clouds, ice, the atmosphere) or absorbed by the atmosphere and the surface
  • radiation emitted by the surface and the atmosphere back into space.

Greenhouse gases (carbon dioxide, methane and water vapour) let most of the Sun’s shorter-wavelength radiation through but absorb some of the longer-wavelength infrared emitted by the Earth. They re-emit it in all directions, including back towards the surface. This makes the Earth warmer than it would be without them. Without an atmosphere, the surface would cool very sharply at night, as the Moon’s surface does.

If the Earth absorbs radiation faster than it emits it, its temperature rises; as it warms it emits more, until a new balance is reached at a higher steady temperature. Adding more greenhouse gases reduces the radiation escaping, so the average temperature rises.

Worked example: (Higher tier) radiation balance

Each square metre at the top of the atmosphere receives on average 340 W from the Sun; 30% is reflected.
reflected = 0.30 × 340 = 102 W; absorbed = 340 − 102 = 238 W per m²
If the Earth emits 238 W per m² into space, the rates balance.

Answer: average temperature stays constant

Common mistakes

  • Saying shiny surfaces are good emitters because they “shine”. Shiny surfaces are the worst emitters and absorbers.
  • Saying greenhouse gases “trap heat” or “reflect” infrared. They absorb longer-wavelength infrared and re-emit it.
  • Forgetting that the Earth’s surface also emits radiation all the time, day and night.
  • Thinking an object in sunlight keeps warming for ever. It warms until emission equals absorption.

Revision questions

Try these, then check your answers below each one.

Question 1. Which surface is the best absorber of infrared radiation? (Choose from: matt black, shiny silver, glossy white, shiny black.)

Answer: matt black

Working: Matt black is the best absorber and the best emitter; shiny silver is the worst.

Question 2. True or false? Greenhouse gases reflect infrared radiation back to the Earth.

Answer: False

Working: False. Greenhouse gases absorb longer-wavelength infrared from the Earth and re-emit it, some back towards the surface.

Question 3. Name one greenhouse gas.

Answer: carbon dioxide

Working: Carbon dioxide, methane and water vapour are greenhouse gases.

Related lessons

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This lesson is section 2.3 (AGCPH0203) 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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