Black Body Radiation

(Separate Physics only) This topic is assessed only in the separate GCSE Physics course, not in Combined Science.

All bodies emit and absorb infrared radiation, no matter what temperature they are. A cup of tea, an ice cube, your own body and a distant star all radiate infrared continuously, and all absorb infrared from their surroundings.

The hotter a body is, the more radiation it emits in a given time.

This is why a radiator feels warm from a distance without touching it: it emits more infrared than the cooler room around it, and that radiation transfers energy to you. The rate of emission rises steeply with temperature, so a small rise in temperature gives a large rise in the radiation emitted.

An object also both emits and absorbs at the same time. Whether it warms up, cools down or stays the same depends on which rate is larger, a point developed later in this lesson.

This is thermal radiation by infrared, which needs no medium and travels through a vacuum, which is how energy reaches the Earth from the Sun.

(Separate Physics only)

To describe emission and absorption exactly, physicists use an idealised object called a perfect black body.

A perfect black body absorbs all of the radiation that hits it. It reflects none and transmits none of the incident radiation.

Because it absorbs everything and reflects nothing, no reflected light reaches your eye from it, which is why the idea is called a "black" body. A matt black surface is a good real-world approximation.

The second, equally important property follows from the first:

A perfect black body is also the best possible emitter of radiation. A good absorber is always a good emitter.

This link between absorbing and emitting explains the results of Required Practical 10 (infrared and surfaces): the matt black face of a Leslie cube both absorbs the most infrared and emits the most, while the shiny silver face absorbs and emits the least. Good absorbers and good emitters are the same surfaces.

(Separate Physics only)

Every body emits a spread of wavelengths, not a single one. The intensity and the wavelength distribution of the radiation emitted both depend on the temperature of the body.

As a body gets hotter, two things happen to the radiation it emits:

1. The total intensity increases — more energy is radiated each second across all wavelengths.
2. The peak of the emission shifts to shorter wavelengths — the wavelength at which the most radiation is emitted becomes shorter.

You can see both effects in a heated piece of metal. A cool object emits mostly long-wavelength infrared, which you cannot see. As it is heated, it begins to glow dull red, then orange, then yellow-white as more of its radiation moves into the shorter-wavelength visible range, and the whole glow gets brighter.

Temperature	Total intensity emitted	Peak wavelength
Cooler	Lower	Longer (infrared)
Hotter	Higher	Shorter (towards visible light)

The colour of a hot object is a clue to its temperature. A blue-white star is hotter than a red one, because its emission peaks at a shorter wavelength.

(Higher Tier only)

Because a body emits and absorbs radiation at the same time, its temperature depends on the balance between these two rates.

• If a body absorbs radiation faster than it emits, its temperature rises.
• If a body emits radiation faster than it absorbs, its temperature falls.
• If a body absorbs and emits at exactly the same rate, its temperature stays constant.

(Higher Tier) A body at a constant temperature is absorbing radiation at the same rate as it is emitting it.

This is a dynamic balance, not a state of no activity: the body is still absorbing and emitting all the time, but the two rates are equal, so there is no overall change in its energy and its temperature holds steady. A change to either rate, for example more incoming radiation, upsets the balance and the temperature changes until a new balance is reached.

(Higher Tier only)

The same radiation balance sets the average temperature of the Earth.

(Higher Tier) The temperature of the Earth depends on the rates at which it absorbs radiation from the Sun, emits radiation back into space, and reflects radiation into space.

During the day the Earth absorbs short-wavelength radiation (including visible light) from the Sun. It also emits longer-wavelength infrared radiation out to space, day and night, and reflects some incoming radiation straight back (for example off clouds and ice). The average temperature stays roughly steady when the rate of emission and reflection into space matches the rate of absorption from the Sun.

If the rate of absorption rises above the rate of emission and reflection, for example if the atmosphere traps more of the outgoing infrared, the balance tips, energy builds up, and the average temperature rises until a new balance is reached.

1. Saying only hot objects emit radiation

Every body emits infrared radiation at any temperature, and every body absorbs it too. Hotter bodies simply emit more per second. Even cold objects like ice emit infrared.

2. Forgetting that a good absorber is a good emitter

A perfect black body absorbs all incident radiation and is the best emitter. Matt black surfaces are best at both; shiny silver surfaces are worst at both. Do not treat absorbing and emitting as opposites.

3. Only mentioning intensity when temperature changes

When a body gets hotter, the intensity rises and the peak wavelength gets shorter. A full answer states both changes, not just that it "emits more".

4. Confusing which way the temperature moves

Temperature rises when a body absorbs faster than it emits, and falls when it emits faster than it absorbs. At constant temperature the two rates are equal. State the comparison of rates, not just "it gains energy".

5. Leaving reflection out of the Earth's balance

The Earth's temperature depends on radiation absorbed from the Sun, radiation emitted to space, and radiation reflected back into space. Omitting reflection gives an incomplete Higher Tier answer.