Black Body Basics

Black Body Basics

Point a thermal camera at anything at all, a kettle, a wall, a person, a bag of frozen peas, and it glows. Every object emits infrared radiation, all the time, and absorbs it at the same time. Temperature changes two things about what an object emits: • How much. The hotter a body is, the more infrared it radiates every second. • Which wavelengths. As a body gets hotter, the radiation shifts towards shorter wavelengths. That is why a piece of steel in a forge glows dull red, then orange, then white as it heats: the peak is moving through the visible range. Nothing here is optional or occasional. A cold object emits less, not none.

From red to white

A blacksmith heats a bar of steel. It first glows dull red, then orange, and finally white. What is happening as the temperature rises?

• It radiates more energy every second, and the radiation shifts towards shorter wavelengths, so more of it falls in the visible range
• It radiates more energy every second, but the wavelengths are unchanged
• The radiation shifts towards longer wavelengths as it gets hotter
• It stops emitting infrared once it is hot enough to glow visibly

Surfaces matter

Required Practical 10 compares surfaces, and the result is one rule with two halves. A good absorber is also a good emitter, which sounds surprising until you notice that both are the same question about how readily a surface exchanges radiation:

Which are true?

Select ALL THREE statements about infrared radiation that are TRUE.

• Every body emits and absorbs infrared at the same time, whatever its temperature
• A surface that absorbs infrared well also emits it well
• The surface that heats up fastest in the sun is also the one that cools down fastest at night
• Only hot objects emit infrared; cold ones do not
• Shiny surfaces are the best absorbers of infrared
• Temperature has no effect on how much a body radiates

Why is it that colour?

Keeping tea hot

Two identical pots of tea, one matt black and one shiny silver, are left on a table. Reason it through.

• Which pot emits infrared radiation faster?
• So which pot of tea cools down more quickly?
• The pots are moved into bright sunshine instead. Which now warms up faster?
• You want to keep a drink warm for as long as possible. Which surface, and why?

The car park

Two identical cars stand in a car park all day, one matt black and one shiny silver. The black one is far hotter inside by the afternoon. Overnight, which cools down faster?

• The black one again, because the surface that absorbs best also emits best
• The silver one, because it did not get as hot to begin with
• Both at the same rate, since colour only affects absorption
• Neither cools, because there is no sunlight at night to drive the process

The perfect black body

Physicists find it useful to imagine an ideal object: a perfect black body, which absorbs all the radiation that falls on it, reflecting and transmitting none. By the rule you already have, a perfect absorber must also be a perfect emitter, so a perfect black body is the best possible emitter at any given temperature. Nothing real is quite perfect, and stars come remarkably close, which is why the model is worth having. It is also the reason the topic is called black body radiation even though the objects concerned are frequently white-hot.

A balancing act

Every object is absorbing and emitting at once, so what matters is which of the two is winning:

A century of warming

Measurements show that a planet's average temperature has been slowly rising for a hundred years. What must that mean about its radiation?

• Over that period it has been absorbing radiation slightly faster than it has been emitting it
• It has been emitting faster than it absorbs
• Absorption and emission have been exactly equal throughout
• It has stopped emitting radiation altogether

The Earth's balance

The Earth absorbs radiation from the Sun, mostly at short wavelengths, and emits infrared back out to space. For most of history those two rates have been close to equal, and the temperature has been correspondingly steady. Greenhouse gases in the atmosphere absorb some of the outgoing infrared and re-emit it, and part of that goes back downwards. The result is that less of the outgoing radiation escapes. Absorption from the Sun has not changed, and emission to space has fallen, so the balance is broken in the direction that means warming. The temperature then rises until the Earth is hot enough to emit fast enough to balance again, at a new and higher temperature. That last sentence is the one worth learning. The system does not simply heat up for ever; it settles at a new balance point, and the question is where.

Why a planet warms

Put the steps in order to explain how greenhouse gases raise the Earth's temperature.

• The Earth absorbs radiation from the Sun
• The Earth emits infrared radiation back out towards space
• While the absorbed and emitted rates are equal, the temperature stays constant
• Greenhouse gases build up and absorb some of the outgoing infrared
• Less radiation escapes, so the Earth now absorbs more than it emits and warms

Explain the blanket

Exam practice. In about 50 words, explain why a survival blanket given to someone with hypothermia is made of shiny material. Include:

• the property a shiny surface has, in terms of emitting infrared
• what that does to the heat the person's body is radiating away
• why a matt black blanket would be a worse choice

Radiation summary

Every object emits and _____ infrared radiation continuously, and the hotter it is the more it radiates each second, at _____ wavelengths. Matt black surfaces are the best absorbers and, by the same rule, the best _____. A body stays at a constant temperature only when what it absorbs equals what it emits; if it absorbs more than it emits, its temperature _____.