Surface Colour and Heating

Most people say you shouldn’t go out on a summer’s day wearing black because you will get too hot. It would be much better to wear white as this will keep you cooler. But is that actually true?

We can test this idea using two boiling tubes that have been sprayed to have different surface colours; one is dull black and the other is shiny silver (equivalent to super-bright white). The two tubes are filled with the same amount of water and are placed the same distance to opposite sides of a Bunsen burner flame (heat source). The temperature of the water in each tube will be measured at regular intervals using a thermometer and a clock respectively.

If we plot the results obtained, we will get a graph similar to the one shown below.

The heating curve for the dull black tube is always above the curve for the silver tube, showing that the black surface absorbs heat energy faster than the silver surface.

You can also see that both curves are becoming flatter. This shows that both tubes are reaching a stable temperature and that, no matter how long the experiment is left running, the black tube will always be hotter than the silver tube.

This last point is particularly important: black surfaces don’t just absorb heat faster, they also reach a higher maximum temperature than silver surfaces.

We can extend our list of surface colours to include the effect of surface texture. The black surface was deliberately a dull finish and the silver surface was deliberately shiny. We could have included a shiny black surface and a dull silver surface – as well as white surfaces in both dull and shiny versions. If we had done all six variations, the order of heating would have looked like this;

An alternative method for investigating this effect, which you must also learn, is given on p86 of your CGP Complete Revision & Practice book.

The explanation for these observations is that heat energy can be transferred using infra-red radiation (a type of electromagnetic wave) and that dull and dark surfaces absorb this energy at a faster rate than shiny and light surfaces. The energy that is not absorbed by shiny, light surfaces is reflected away from the object.

Never, ever say that dark surfaces attract heat. The word “attract” suggests that there is a force at work, like opposite poles of a magnet attracting each each. There is no force involved here. The effect is simply one of efficiency: dull black surfaces are more efficient at absorbing infra-red radiation, whereas shiny silver surfaces are more efficient at reflecting the same radiation.