(c) Anthony K. Grafton 2003

Hot Colors, Cool Cars

Have you ever noticed that after cars have been outside in the sun, those painted dark colors get hotter than those painted white? Even if you wear a dark shirt during the day this summer, you’ll feel a noticeable difference in temperature from someone wearing a white shirt. It all has to do with how much light is absorbed due to the coloring.

Objects can show color by either reflecting or transmitting light. Things like paint, clothing, bricks, and leaves all have particular hues due to the particular colors of light that they absorb and reflect. Black objects, and other things with very dark colors, absorb most of the light that strikes their surfaces. Light colors such as white reflect most of the light that strikes them. Light from the sun has a lot of energy, and the more light that is absorbed, the higher the temperature of the object gets. So black cars get hotter in the sun than do white cars.

Color can also result from the transmission of light, rather than reflectance. Colored glass, for example, allows certain light to pass through, while the rest of the light is absorbed. So the covers on traffic stop-lights absorb all the colors except red light, which is able to go through the glass or plastic. Compare this with leaves, which get their color by absorbing nearly every color of visible light except green, which is reflected.

Ask your parents to help you try this experiment: put four glasses of water in a sunny window. To three of the four glasses, add increasing amounts of red or blue food coloring (perhaps two drops, six drops, and twelve drops). After the glasses have been in the sun for a couple of hours, which one is hotter, the light ones or the dark ones? E-mail your results to Science Corner.