MadSci Network: Other

Re:Why is a black car hotter than a white car?

Date: Tue Feb 17 09:23:04 1998
Posted By: Jocelyn Wishart, Lecturer, Education, Loughborough University
Area of science: Other
ID: 885839260.Ot

Light itself is made up of the entire spectrum of colours as seen in a rainbow. We see different colours because different substances either absorb, transmit or reflect some of these component colours in differing amounts. For instance a substance like chlorophyll found in green plants absorbs (takes out) the violet and red light from each end of the spectrum leaving green light to be reflected back to our eyes.

Each substance itself is made up of tiny, tiny particles called atoms. Each atom contains even tinier particles called electrons surrounding its centre or nucleus. The electrons clouding round the nucleus of an atom have different amounts of energy, some orbit at higher energy levels than others.

When a substance absorbs a particular colour from the light falling on it, the amount of energy in that colour of the light is exactly the same as the difference between two of those energy levels for its orbiting electrons. The electrons in the lower level use the coloured light's energy to jump into the higher level and that colour of light gets taken out (absorbed).

A car painted black absorbs from all the colours in the sunlight and so its electrons take in much more of the light's energy to move themselves to higher energy levels than a car painted white which reflects the light. We can feel these higher energy levels, black cars feel hotter to us.

Administator's note:
This is a good description of what happens when a white and black object are both in the sun, but the reverse will be true at night. Black objects generally radiate away energy faster than white ones, so they cool faster. Notice that heat sinks on electronic compoments are generally painted or treated black. DW.

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