Edexcel A Level (IAL) Physics-5.37 Black Body Radiation- Study Notes- New Syllabus

Black Body Radiator and Interpretation of Radiation Curves

A black body radiator is an idealised object used to model the emission of thermal radiation.

What Is a Black Body Radiator?  

A black body is a body that:

• Absorbs all electromagnetic radiation incident on it.
• Reflects no radiation.
• Emits the maximum possible radiation at any given temperature.

Important:

• A black body is a perfect absorber and perfect emitter.
• Real objects approximate black body behaviour.

 Emission of Radiation by a Black Body

The radiation emitted by a black body depends only on:

• Its temperature.
• Not on its shape or material.

This radiation is emitted over a continuous range of wavelengths.

Radiation Curves for a Black Body

A radiation curve is a graph of:

• Intensity (or power per unit area per wavelength)
• Against wavelength

Each curve corresponds to a different temperature.

Key Features of Black Body Radiation Curves

• Each curve has a single peak.
• The peak occurs at a wavelength called the peak wavelength.
• As temperature increases, the peak moves to shorter wavelengths.
• The total area under the curve increases with temperature.

Key ideas:

• Hotter objects emit more radiation.
• Hotter objects emit radiation of shorter wavelengths.

 Effect of Increasing Temperature

When temperature increases:

• The peak intensity increases.
• The peak wavelength decreases.
• The total energy emitted increases rapidly.

This explains why:

• Cool objects glow red.
• Hot objects glow white or blue.

 Interpreting Radiation Curves

• The curve with the highest peak corresponds to the highest temperature.
• A shift of the curve left means an increase in temperature.
• The area under the curve represents total power emitted.

Important distinction:

• Peak height ≠ total energy.
• Total energy depends on the entire area under the curve.

 Real Black Body Examples

• A cavity with a small hole acts as a near-perfect black body.
• The Sun behaves approximately as a black body.
• Stars can be analysed using black body radiation curves.