Every object emits and absorbs infrared
Infrared (IR) is a type of electromagnetic wave. We cannot see it, but we feel it as warmth. Every object emits infrared, whether it is a hot iron, your body or an ice cube. Every object also absorbs infrared that falls on it.
Hotter objects emit more
The hotter an object is, the more radiation it emits every second (more power). The spread of wavelengths also shifts: the strongest radiation moves to shorter wavelengths. So a warm object gives out only infrared, but a very hot object also gives out visible light: first dull red, then orange, then yellow-white. The Sun's surface (about 5,800 K) is hot enough that its peak is in visible light.
Which surfaces absorb and emit best?
Dark, matt (dull) surfaces are the best absorbers and the best emitters. Light, shiny surfaces are the worst absorbers and worst emitters; they are good reflectors. You can test this with a Leslie cube: a hollow metal cube filled with hot water, with four faces painted differently (matt black, shiny black, matt white, shiny silver). An infrared detector held the same distance from each face shows the matt black face emits the most.
Try it: on a sunny day, put two identical glasses of water in the sun, one wrapped in black paper and one in aluminium foil. After 30 minutes, which feels warmer? (The black one, because it absorbs more.)
What is a perfect black body?
A perfect black body is an imaginary object that absorbs all the radiation falling on it. It does not reflect any, and none passes through it. Because good absorbers are also good emitters, a perfect black body is also the best possible emitter at any temperature.
The radiation it gives out is called black body radiation. It depends only on the temperature, not on what the object is made of. That makes it a useful 'ideal' to compare real objects with.
Nothing real is perfect, but a hollow box with a tiny hole comes close: radiation entering the hole bounces around inside and is absorbed bit by bit, so almost nothing comes back out. Stars, including the Sun, behave roughly like black bodies too.
Temperature balance and the Earth
Whether an object warms up, cools down or stays the same depends on two rates:
- If it absorbs faster than it emits, its temperature rises.
- If it emits faster than it absorbs, its temperature falls.
- If both rates are equal, its temperature stays constant.
As a warming object gets hotter it emits more, so it naturally settles at a steady temperature where in = out.
The Earth's temperature
The Earth absorbs radiation from the Sun (mostly visible light and some infrared and ultraviolet). Some sunlight is reflected straight back by clouds, ice and snow. The warmed ground then emits infrared. Some of this infrared escapes to space, but gases such as carbon dioxide, methane and water vapour absorb part of it and send it out again in all directions, including back down. This keeps the Earth warmer than it would be with no atmosphere. The Earth's temperature depends on the balance between incoming and outgoing radiation, so changes to clouds, ice or these gases change the temperature.
Key formulas and definitions
- All objects emit and absorb infrared radiation at every temperature.
- Higher temperature β more radiation per second and a shorter peak wavelength.
- Good absorber = good emitter (dark matt). Poor absorber = poor emitter = good reflector (light shiny).
- Perfect black body: absorbs 100% of incident radiation; best possible emitter.
- Absorb rate > emit rate β temperature rises; equal β constant; less β falls.
Worked examples
1. A cup of tea is left in a room. Its temperature falls and then stays at room temperature. Explain using radiation.
At first the hot tea emits radiation faster than it absorbs it from the cooler room, so it cools. As it cools it emits less. When it reaches room temperature it emits and absorbs at the same rate, so its temperature stays constant.
2. Why are rooftop water tanks often black?
A dark matt surface is a good absorber of radiation, so the tank absorbs more energy from sunlight and the water heats up faster.
3. A metal rod in a furnace glows red; when hotter it glows yellow-white. What does this show?
As temperature rises, the object emits more radiation and the peak wavelength gets shorter, so the colour shifts from red (longer) towards yellow and white (more shorter wavelengths).
Common mistakes
- Thinking only hot objects emit infrared. Every object emits infrared; cold ones just emit less.
- Saying a black body must look black. A very hot black body glows brightly (stars are near black bodies).
- Thinking shiny surfaces are good emitters because they look bright. Shiny surfaces reflect well but emit poorly.
- Believing a constant temperature means no radiation. It means absorption and emission are equal, not zero.