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Name: Edward
Status: student
Age: 9
Location: N/A
Country: N/A
Date: 1999-2001


Question:
I am doing a demonstration of the fact that black and darker colors absorb more heat than white and lighter colors. I am using felt in colors of the rainbow (ROYGBIV). I wanted to know why, at the molecular level, this happens. From what I have read it seems that colors get warmer as you go from longer to shorter wavelengths because the longer wavelengths are closer to the infrared wavelength and so can produce or transmit(?) more heat than the shorter wavelengths. So, a red cloth will be cooler than a violet cloth when shined on with a light for a given amount of time because the red cloth is reflecting the red wavelength--and absorbing all the other ones--which gives off more heat than the violet cloth which reflects violet (a shorter wavelength which is cooler). Is this accurate? If it isn't what's the real answer? Thank you for replying ASAP. Edward


Replies:
Edward,

The simple answer to your question is that lighter colors have a greater albedo...that is, they reflect more light. Darker colors absorb comparatively more light. Since light is energy, an absorption would increase a materials temperature. Note that an object appears white if it reflects all colors, black if it absorbs all colors. Naturally there are gradients of color and therefore gradients of absorption. So, too, the type of material will affect its relative heating. For a given material, however, this describes in simple terms the phenomenon you are seeing.

Thanks for using NEWTON!
Dr. Rupnik


It's true that red and orange are regarded as warmer colors than blue and violet, but this describes the way people feel when they look at colors, and it has nothing to do with the abilities of different colors to warm things up. Actually a blue photon contains more energy than a red one, and so if totally absorbed it would be converted into more heat.

In your demonstration, the only thing that matters is how much light is absorbed, how much is reflected, and how much is radiated away. The darker the color, the less visible light it is reflecting and the more it is absorbing. The absorbed light is converted to heat.

There is a lot of missing information here. Sunlight contains a wide range of "colors" not all of which you can see. There could be two identical looking pieces of felt one of which absorbs infrared radiation,the other of which reflects it. The absorber would get warmer.

Tim Mooney



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