Black Body Radiation Revision Notes for AQA GCSE Physics

Black body radiation

What is black body radiation?

All objects above absolute zero temperature give off infrared radiation. This is called black body radiation. The key thing to remember is that the hotter an object gets, the more infrared radiation it emits.

Think about it like this: even a cup of tea at 90°C gives off radiation that we can't see. But the Sun at 5700°C gives off so much radiation that some of it becomes visible light!

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The term "black body" doesn't mean the object is actually black in colour. It's a theoretical concept used in physics to describe perfect absorbers and emitters of radiation.

Perfect black body

A perfect black body is a special type of object that has very specific properties when it comes to radiation. Understanding these properties is essential for understanding how radiation works in the real world.

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A perfect black body represents an idealised object with these four key characteristics:

Absorbs all radiation that hits it
Reflects no radiation at all
Transmits no radiation through it
Is also a perfect emitter of radiation

This means a perfect black body is the best possible absorber AND the best possible emitter of radiation. In reality, no object is a perfect black body, but some objects come close to this ideal.

Factors affecting temperature

There are three important rules about how objects and radiation interact. These rules help us understand why objects gain or lose heat through radiation:

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Real-World Example: Understanding Temperature Balance

Consider a metal roof on a sunny day:

Morning: The roof absorbs more solar radiation than it emits → temperature increases
Midday: The roof reaches equilibrium → absorbs = emits → steady temperature
Evening: No more solar input, roof emits more than it absorbs → temperature decreases

Steady temperature: An object stays the same temperature when it absorbs the same amount of radiation as it emits.
Temperature increases: An object gets hotter if it absorbs more radiation than it emits.
Temperature decreases: An object gets cooler if it emits more radiation than it absorbs.

Intensity and wavelength

Intensity means the energy given off per square metre every second. It's basically how much radiation power an object produces, measured in watts per square metre ( $W/m^2$ ).

As objects get hotter, several important changes occur in their radiation characteristics:

The intensity increases (more radiation given off)
The wavelength of maximum intensity gets shorter
More types of radiation are produced (infrared, visible light, and UV)

This relationship explains why hot objects can glow different colours! The peak wavelength follows Wien's displacement law: $\lambda_{max} = \frac{b}{T}$ , where $b$ is Wien's displacement constant.

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Wien's displacement law shows us that as temperature increases, the peak wavelength of emitted radiation becomes shorter. This is why a piece of metal glows red when heated to about 800°C, but appears white-hot at much higher temperatures.

Why stars have different colours

Stars are excellent examples of black body radiation in action. The colour of a star depends on its surface temperature, which demonstrates the relationship between temperature and the wavelength of emitted radiation.

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Stellar Colour Examples:

Cool Red Stars (3,000K): Like Betelgeuse

Peak emission in longer wavelengths (red/infrared)
Surface temperature around 3,000K

Hot Blue Stars (30,000K): Like Rigel

Peak emission in shorter wavelengths (blue/UV)
Surface temperature around 30,000K

Our Sun (5,700K): Yellow-white star

Peak emission in visible light range
Appears white when viewed from space

Cooler stars appear red (longer wavelengths)
Hotter stars appear blue-white (shorter wavelengths)

This happens because changing the temperature changes the wavelength of the electromagnetic radiation, which changes the colour we see. The relationship follows Planck's law for black body radiation.

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Key Points to Remember:

All objects above absolute zero emit infrared radiation
Hotter objects emit more radiation than cooler ones
Perfect black bodies absorb all radiation and are also perfect emitters
An object's temperature depends on the balance between radiation absorbed and emitted
Star colours depend on surface temperature - hotter stars are blue-white, cooler stars are red
Wien's displacement law explains why peak wavelength decreases as temperature increases

Black Body Radiation (AQA GCSE Physics): Revision Notes