(by Amara Graps)
The subject of the perception of colors has many facets- thepsychological aspects, the physiological aspects, the physicalaspects, among others.
Color is an interesting and complex subject. If you would like tounderstand colors in a complete way, I suggest starting with PaulHewitt's _Conceptual Physics_ Chapter 27: "Color", and then browsingthrough: M. Minnaert's _The Nature of Light and Color in the OpenAir_ (A Dover Press book).
Also, in your question, you are talking as if the Sun is appearingorange in color. I'm assuming that you are seeing it with your nakedeye, such as a setting Sun, low on the horizon, rather than seeing theSun through an orange filter. (Note that it is dangerous to lookdirectly at the bright Sun!)
What I say below is from Paul Hewitt's _Conceptual Physics_.
Strictly speaking, the colors that you see are not in the emittingsubstances, or even in the light that they reflect because color is a*physiological* experience and therefore dependent on the individualviewing the object. Color is in your head. So an apple *appears* red,the setting Sun *appears* orangish-red, and so on.
The physics of colors is the following. The colors that we seedepends on the frequency of the emitted light. Different frequenciesof light are perceived as different colors: the lowest frequency beingred, the highest violet, and in-between those two are the infinitenumber of hues that make up the color spectrum of the rainbow. Thefrequency of the emitted light depends on electron transitions betweenthe atomic energy levels of the source. However, most objects_reflect_ rather than _emit_ light, and they reflect only part of thelight that is shining on them- the part that gives them their color.
Your question points out differences in color one sees due to emitted and reflected and transmitted light.
An apple appears red because in white light, the apple _reflects_ thered part of the white light. (Try looking at an apple using aprism. If you pass sunlight through a prism in order to generate aspectrum, and hold the apple in each of the dominating colors, youwill notice the color of the apple changing.))
We see the Sun however, mostly as an _emitting_ and _transmitting_object through our Earth atmosphere. These two factors make ourperception of the Sun's colors as a different phenomena than lightreflection of an object such as a red apple.
The Sun *emits* white light, which is actually a composite of all ofthe visible frequencies of light. (A prism shows you the visiblefrequencies of white light.) The distribution of solar frequencies isuneven, the most intense frequency is in the yellow-green part of thespectrum. All of the visible frequencies mixed together producewhite. (Note that this white also results from the combination of onlyred, green, and blue light (the additive primary colors).)
Our Earth atmosphere predominately contains nitrogen and oxygenmolecules. These atoms behave as behave like tiny optical tuning forksand selectively scatter light waves of appropriate frequencies. Thenatural frequencies that nitrogen and oxygen resonate with thesunlight at are in the ultraviolet part of the white light solarspectrum. Visible violet light has a frequency close to theultraviolet frequencies, so there is considerable forced vibrations,and therefore, violet light scatters in our atmosphere in largeamounts. For every ten violet photons scattered from a sunlight beam,only one red photon in scattered. Some blue light, and a little bit ofgreen light, scatter in similar ways, so that is why the sky appearsblue.
Red, orange and yellow light are lower frequencies of light than blueand green. These lower frequencies are scattered the least by nitrogenand oxygen. Therefore, red, orange, and yellow light are transmittedthrough our atmosphere more readily than blue. (Red, which isscattered the least amount, traverses more atmosphere relativelyunhindered than any other color.) Do you see now that we are notseeing the effects of reflection, like from an apple? We are seeingthe consequences of emitted and transmitted light.
Therefore when we see the Sun setting, a white-light-emitting object,through our thick Earth atmosphere, the higher frequencies arescattered while the lower freqencies are transmitted. At sunset, whenthe Sun is lowest in the sky, and seen through the largest amount of"air mass", we see it as an orangish-red color.
You may also enjoy this article on The Color of the Sun
