Revision notes · Waves

Black body radiation (physics only)

Emission and absorption of infrared radiation4.6.3.1

All objects, regardless of their temperature, continuously emit and absorb infrared radiation. The hotter an object is, the more infrared radiation it radiates in a given time (it's more powerful), and the more of that radiation is at shorter wavelengths.

Definition: An object's temperature depends on the balance between the radiation it absorbs and the radiation it emits: if an object absorbs radiation at a greater rate than it emits it, its temperature increases; if it emits radiation at a greater rate than it absorbs, its temperature decreases; if the two rates are equal, its temperature stays constant.

⚠️ Common mistake: A body at a constant temperature is NOT radiation-inactive — it is still continuously absorbing and emitting radiation, just at equal rates, so there's no net change in its energy or temperature.

Emission and absorption of infrared radiation

Investigating infrared radiation: a Leslie's cube — a hollow cube with a different surface finish on each face (matt black, dull white, shiny grey, shiny silver) — is filled with hot water, so all four faces reach the same temperature. An infrared detector held at the same distance from each face in turn measures how much radiation it emits. The matt black face emits the most, and the shiny silver face the least — confirming dark, matt surfaces are the best emitters (and absorbers) of infrared radiation, while light, shiny surfaces are the worst.

Perfect black bodies and radiation4.6.3.2

Definition: A perfect black body is an object that absorbs all of the electromagnetic radiation incident on it — it reflects and transmits none. A perfect black body is also the best possible emitter of radiation at any given temperature.

The Earth's temperature is determined by the balance between the radiation it absorbs from the Sun and the infrared radiation it emits back out into space: most incoming solar radiation is absorbed by the Earth's surface and atmosphere (with some reflected straight back), while the Earth itself constantly radiates infrared energy outward. The Earth's overall temperature stays roughly stable because, averaged over time, the rate of energy absorbed is balanced by the rate of energy emitted.

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