Blackbody Radiation Basics: Why Hot Things Glow in Colour

A blackbody is an ideal object that absorbs all radiation falling on it and emits radiation purely because of its temperature. Real hot objects are not perfect blackbodies, but stars, molten metal, and glowing coal behave closely enough that blackbody laws predict their colour and brightness well.

This topic explains why heated iron first looks dull red, then orange, yellow, and almost white as temperature rises. It also lets astronomers find a star temperature from its colour alone, without touching it.

Key Concepts

Thermal radiation is electromagnetic radiation emitted due to temperature. Spectrum means how intensity is spread over wavelength. Peak wavelength is where emission is strongest. Colour depends on peak and on how much visible light is emitted. At room temperature the peak is in infrared, so objects do not glow visibly but can be seen by thermal cameras. The Sun surface is about 5800 kelvin and peaks near green-yellow, though combined light looks white.

Formulas

Wien displacement law: lambda_max x T = b, where lambda_max is peak wavelength in metre, T in kelvin, b = 2.898 x 10^-3 metre kelvin. Stefan-Boltzmann law: P = sigma A T^4, power per? Total emitted power P in watt, sigma = 5.67 x 10^-8 watt per square metre per kelvin^4, A area in square metre, for a perfect blackbody emissivity 1. For real surfaces multiply by emissivity between 0 and 1. Both laws assume thermal equilibrium.

Worked Example

A glowing filament has temperature 3000 kelvin. Find its peak wavelength and colour sense.

Step 1: lambda_max = b / T = 2.898 x 10^-3 / 3000 = 9.66 x 10^-7 metre = 966 nanometre. This peak is in infrared, but the visible tail contains strong red, orange, and yellow, so filament looks warm white-yellow. Step 2: compare Sun: lambda_max = 2.898 x 10^-3 / 5800 = 5.0 x 10^-7 metre = 500 nanometre, in visible range, matching sunlight brightness in visible light.

Common Misconceptions

• Blackbody must look black: Wrong. A blackbody at high temperature glows brightly; black refers to perfect absorption at any temperature.
• Red hot is hotter than white hot: Wrong. White hot is hotter; colour shifts red to orange to white as temperature increases.
• Only the Sun emits radiation: Wrong. Every object above absolute zero emits thermal radiation, mostly infrared when cool.

Real-World Applications

Astronomers use colour and spectrum to measure star temperatures and sizes. Thermal imaging cameras detect infrared blackbody radiation from bodies, machines, and buildings to find fever, faults, or heat leaks. Incandescent bulbs waste much energy as infrared because their peak is outside visible range. Climate science uses blackbody laws to model Earth emission to space.

Blackbody radiation links temperature to colour and total power through Wien and Stefan-Boltzmann laws. As temperature rises, emission grows rapidly and peak wavelength shortens, so hot things glow from red toward white.