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Name: Jessica
Status: educator
Grade: K-3
Location: TX
Country: N/A
Date: 5/10/2005


Question:
I went to a workshop that stated the color of the sun is white. I was told that we see it in yellow-orange because of the way light moves through the Earth's atmosphere. On your web site, it states the sun is yellow-orange. Who is correct? I have been teaching it is white based on my workshop experience. Thank you for clarifying!


Replies:
The complication you are encountering is the difference between "color" a sensory response and "spectral distribution" a physical measurement. The "average" temperature of the Sun's surface is about 6000 C. and an object at that temperature would appear to be "white". However, from the appended web site:

http://www.solarviews.com/eng/sun.htm

you see that there is a large variance in the temperature. To further complicate matters, as we view the Sun from the surface of the Earth there are obviously different colors depending upon atmospheric conditions. Sunrise and sunset both make the Sun appear yellow, orange and/or red to varying degrees. If an object were heated to 6000 C. in a room or furnace it would appear "white hot". But in that case there would not be a blanket of air to alter the "spectral distribution". The light from the Sun when spread out by a prism breaks up into violet, blue, green, yellow, orange, and red. Even that does not include ultraviolet and infrared. So what color would the "color" of the Sun be? It is all of them -- all at the same time -- but none of them at once.

Vince Calder


Dear Jessica,

The sun as seen in outer space is quite white. It emits different wavelengths with intensities given by the law of blackbody radiation. It may feel a little strange to think of the sun as a black body. It just means that the sun absorbs all the light striking it and is an equally good radiator of electromagnetic radiation. (Good absorbers must also be good radiators or they would get hotter than their surroundings).

The maximum intensity of light emitted by black body radiation is at a wavelength given by the Wien Displacement Law which says: Wavelength of maximum intensity in meters times temperature in Kelvin = 0.0029. Since the temperature of the surface of the sun is around 5800 K, the sun emits light of wavelength 500 nm with the greatest intensity. Since visible light extends from around 400 nm (purple) to 700 nm (red), all of which are emitted by the sun with almost equal intensity, the sun looks white to our eyes.

In passing through our atmosphere, the shortest wavelengths (blue /purple) are scattered most strongly. This answers one of the best known questions in all of physics: "Daddy, why is the sky blue?" It also explains why sunsets (and sunrises) tend to be reddish; much of the blue light is scattered out after the sunlight has travelled through so much atmosphere leaving a preponderance of red.

As might be expected, the answer to your question is a little complicated and both the workshop and the web site are roughly correct.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



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