Dull black surfaces emit infrared best, shiny silver ones worst
Definition
Good emitter
A surface that gives out infrared radiation at a high rate for its temperature, so an object with such a surface cools more quickly than an otherwise identical one.
- Take two identical hot objects at the same temperature and the one with the dull black surface loses energy by radiation faster and cools sooner.
- The one with the shiny silver or white surface is the poorest emitter, and stays hot for longest.
- Both parts of the description matter: colour (black rather than white) and texture (dull and rough rather than smooth and polished).
- This is why the cooling fins on the back of a fridge and the pipework of a car radiator are usually finished in matt black: the job of both is to get rid of energy quickly.
- It is also why a teapot with a polished silver surface keeps tea hot for longer than a matt dark one of the same size.
Practical
Investigation: comparing infrared emission from dull and shiny surfaces
- Aim: to compare how quickly a dull black surface and a shiny surface emit infrared radiation.
- Apparatus: eye protection, two matching thermometers with one bulb painted dull black, a beaker of hot water, two stands and clamps, and a stopwatch.
- Method:
- Stand both thermometers in the same beaker of hot water until they read the same steady temperature.
- Lift both out at the same moment, clamp them side by side in the air, and start the stopwatch.
- Record both temperatures every 30 s30\ \text{s}30 s for several minutes, reading each thermometer at eye level.
- Measure and plot: tabulate time / s against temperature / °C for each bulb, then plot both cooling curves on the same axes with temperature on the yyy-axis.
- Result: the dull black bulb cools faster, so its curve falls more steeply, showing that a dull black surface is the better emitter of infrared.
- Watch out:
- Both thermometers must start at the same temperature and be removed together, or the comparison means nothing.
- Shield the apparatus from draughts, because a breeze across one bulb cools it by convection and hides the radiation effect you are testing.
- Keep the two bulbs the same size and the paint layer thin, so the only difference between them is the surface.
- Safety: wear eye protection, and handle the hot water carefully since it can scald.
The best emitters are also the best absorbers
Definition
Good absorber
A surface that takes in a high proportion of the infrared radiation falling on it instead of reflecting it away.
- A dull black surface is both the best emitter and the best absorber, so the same surface that sheds energy fastest also soaks it up fastest.
- A shiny silver or white surface is the worst absorber, because it reflects most of the radiation that reaches it.
- Every surface splits the radiation arriving at it three ways, into the part absorbed, the part reflected and, for a transparent material, the part transmitted straight through.
- This is why a black car left in a car park on a July afternoon is unbearable inside while a white one nearby is merely warm.
- Designers exploit the rule in both directions: solar panels are matt black to absorb as much as possible, while fuel storage tanks are painted silver to reflect the Sun and stay cool.
- You can show the absorbing effect directly by fixing a drawing pin to the back of each of two identical metal plates with a blob of wax, one plate dull black and one shiny.
- Stand a radiant heater exactly midway between the two plates so each receives the same radiation.
- The wax on the dull black plate melts first and its pin drops, because that plate has absorbed the radiation faster.
Example
- A survival blanket handed out at the end of a marathon is a thin sheet of shiny silver plastic.
- The shiny inner surface is a poor emitter, so it radiates very little of the runner's energy away.
- It also reflects the infrared the runner emits straight back towards them instead of letting it escape.
- A dull black blanket of the same thickness would be a far worse choice, because it would radiate that energy away into the cold air.
Common Mistake
- Do not say a black surface absorbs well but emits badly, because the same surface is good at both.
- Do not mention only the colour, because texture carries a mark of its own and the wording wanted is "dull black" against "shiny silver".
- Do not explain a shiny surface by saying it "does not take in" radiation without saying what happens instead, because the missing word is reflected.
- Do not forget to keep the two objects identical in every other way when you describe a comparison, because otherwise the result proves nothing.
Exam technique
- Use the full phrase dull black or shiny silver every time, since "black" alone often only earns half of what is on offer.
- In an experiment answer, state the one thing you change and at least two things you keep the same, then say what you would observe.
- Read whether the question is about emitting or absorbing, because the surface that wins is the same but the reasoning runs the opposite way.
- Where a real object is named, say what its surface is for, such as a fridge's black fins being there to lose energy quickly.
Self review
- Which type of surface is the best emitter of infrared radiation?
- What happens to most of the radiation that reaches a shiny silver surface?
- Describe an experiment to show which of two surfaces is the better emitter.
- Describe an experiment to show which of two surfaces is the better absorber.
- Why is a solar panel matt black but a fuel storage tank silver?
