top of page

2.2 Infrared Radiation

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

A free GCSE Physics lesson for AQA (8463) from OBA School: infrared as part of the electromagnetic spectrum, emission and absorption by all objects, and black body radiation (physics only). 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 what infrared radiation is and how it can be detected
  • State that all objects emit and absorb infrared radiation, and that hotter objects emit more each second
  • Explain what is meant by a perfect black body and black body radiation
  • Describe how the intensity–wavelength curve of emitted radiation changes as temperature rises
  • (Higher tier) state what happens to the temperature of an object that absorbs radiation faster or slower than it emits it

Key words

  • Infrared radiation: Electromagnetic waves with wavelengths a little longer than red light. We cannot see them but we feel them as warmth.
  • EM spectrum: The family of waves (radio, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays) that travel at the same speed through a vacuum.
  • Emit: Give out radiation.
  • Absorb: Take in radiation, increasing the energy in the object’s thermal store.
  • Perfect black body: An object that absorbs all the radiation that falls on it. It reflects none and transmits none.
  • Black body radiation: The radiation emitted by a perfect black body. No object emits more at a given temperature.
  • Intensity: How much energy the radiation carries each second, per square metre.

What is infrared radiation?

Infrared (IR) radiation is part of the electromagnetic spectrum. Its wavelengths are longer than visible light, just beyond the red end of the spectrum. Like all electromagnetic waves it can travel through a vacuum, so it reaches us from the Sun across empty space. The atmosphere lets most of the Sun’s visible light and infrared through.

You can show that infrared is there by splitting sunlight with a prism and moving a thermometer with a blackened bulb (a good absorber) across the spectrum. The reading rises most just beyond the red, where nothing can be seen.

Key fact: Infrared is not “heat”. It is electromagnetic radiation that transfers energy; the energy raises the temperature of whatever absorbs it.

Every object emits and absorbs infrared

All objects emit and absorb infrared radiation, whatever their temperature – even an ice cube. The hotter an object is, the more infrared radiation it emits in a given time.

  • Thermal imaging cameras detect infrared. Warm bodies show up brightly against cooler surroundings, so they are used for search and rescue at night and for spotting energy leaks from houses.
  • You can tell a hob or an iron is hot without touching it by holding your hand near it and feeling the infrared it emits.

Worked example: what an infrared image tells you

A drone camera shows a car engine as brighter (more infrared) than the road.
More infrared emitted per second means a higher temperature.

Answer: the engine is hotter than the road

Perfect black bodies

A perfect black body absorbs all the radiation that hits it. It does not reflect any, and it does not let any pass through (transmit). A good absorber is also a good emitter, so a perfect black body is also the best possible emitter. The radiation it emits is called black body radiation.

No real object is a perfect black body, but a matt black surface, or a small hole into a dark box, comes close. Stars behave very nearly like black bodies.

Exam tip: Black body = perfect absorber and perfect emitter. “Black” describes how it absorbs, not the colour it glows when hot.

How the radiation changes with temperature

An object at a constant temperature emits radiation over a continuous range of wavelengths. The intensity is greatest at one particular wavelength (the peak).

When the temperature goes up:

  • the intensity is greater at every wavelength
  • the intensity increases most at shorter wavelengths, so the peak moves to a shorter wavelength.

This explains why a heated metal bar first emits only infrared (invisible), then glows dull red, then orange, then yellow-white as it gets hotter: more and more of the emitted radiation is at the shorter, visible wavelengths. In the same way, red stars are cooler than yellow stars, and blue-white stars are the hottest.

Worked example: comparing three stars

Star A is red, star B is yellow and star C is blue-white.
The hotter the star, the shorter the wavelength of its peak emission.
Blue-white has the shortest peak wavelength; red has the longest.

Answer: hottest C, then B, coolest A

Absorbing and emitting at the same time (Higher tier)

An object is absorbing radiation from its surroundings and emitting its own radiation at the same time. What happens to its temperature depends on which rate is bigger:

  • absorbs faster than it emits → its temperature rises
  • absorbs and emits at the same rate → its temperature stays constant
  • emits faster than it absorbs → its temperature falls.

As an object warms up it emits more radiation, so eventually it emits as fast as it absorbs and settles at a steady temperature. You will meet this again for the Earth in section 2.3.

Common mistakes

  • Saying only hot objects emit infrared. All objects do; hotter ones emit more per second.
  • Saying a black body only absorbs. It is also the best emitter.
  • Thinking the peak moves to a longer wavelength when the temperature rises. It moves to a shorter wavelength.
  • Saying a red-hot object emits only red light. It emits a continuous range, mostly infrared.

Revision questions

Try these, then check your answers below each one.

Question 1. How does the wavelength of infrared radiation compare with visible light? (Choose from: longer than visible light, shorter than visible light, the same as visible light, shorter than ultraviolet.)

Answer: longer than visible light

Working: Infrared lies just beyond the red end of the spectrum, so its wavelengths are longer than visible light.

Question 2. True or false? All objects emit infrared radiation, even very cold ones such as an ice cube.

Answer: True

Working: True. All objects emit and absorb infrared; hotter objects emit more per second.

Question 3. What name is given to an object that absorbs all the radiation that falls on it, reflecting and transmitting none?

Answer: perfect black body

Working: A perfect black body absorbs all incident radiation. It is also the best possible emitter.

Related lessons

Continue in the full course

This lesson is section 2.2 (AGCPH0202) 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.

 

 

 

 

 

 

 

bottom of page