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Color of Sun
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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Update: June 2012
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