Why does the Sun appear orange?
(by Amara Graps)
The subject of the perception of colors has many facets- the psychological aspects, the physiological aspects, the physical aspects, among others.
Color is an interesting and complex subject. If you would like to understand colors in a complete way, I suggest starting with Paul Hewitt's _Conceptual Physics_ Chapter 27: "Color", and then browsing through: M. Minnaert's _The Nature of Light and Color in the Open Air_ (A Dover Press book).
Also, in your question, you are talking as if the Sun is appearing orange in color. I'm assuming that you are seeing it with your naked eye, such as a setting Sun, low on the horizon, rather than seeing the Sun through an orange filter. (Note that it is dangerous to look directly 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 emitting substances, or even in the light that they reflect because color is a *physiological* experience and therefore dependent on the individual viewing 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 see depends on the frequency of the emitted light. Different frequencies of light are perceived as different colors: the lowest frequency being red, the highest violet, and in-between those two are the infinite number of hues that make up the color spectrum of the rainbow. The frequency of the emitted light depends on electron transitions between the atomic energy levels of the source. However, most objects _reflect_ rather than _emit_ light, and they reflect only part of the light 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_ the red part of the white light. (Try looking at an apple using a prism. If you pass sunlight through a prism in order to generate a spectrum, and hold the apple in each of the dominating colors, you will 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 our perception of the Sun's colors as a different phenomena than light reflection of an object such as a red apple.
The Sun *emits* white light, which is actually a composite of all of the visible frequencies of light. (A prism shows you the visible frequencies of white light.) The distribution of solar frequencies is uneven, the most intense frequency is in the yellow-green part of the spectrum. All of the visible frequencies mixed together produce white. (Note that this white also results from the combination of only red, green, and blue light (the additive primary colors).)
Our Earth atmosphere predominately contains nitrogen and oxygen molecules. These atoms behave as behave like tiny optical tuning forks and selectively scatter light waves of appropriate frequencies. The natural frequencies that nitrogen and oxygen resonate with the sunlight at are in the ultraviolet part of the white light solar spectrum. Visible violet light has a frequency close to the ultraviolet frequencies, so there is considerable forced vibrations, and therefore, violet light scatters in our atmosphere in large amounts. For every ten violet photons scattered from a sunlight beam, only one red photon in scattered. Some blue light, and a little bit of green light, scatter in similar ways, so that is why the sky appears blue.
Red, orange and yellow light are lower frequencies of light than blue and green. These lower frequencies are scattered the least by nitrogen and oxygen. Therefore, red, orange, and yellow light are transmitted through our atmosphere more readily than blue. (Red, which is scattered the least amount, traverses more atmosphere relatively unhindered than any other color.) Do you see now that we are not seeing the effects of reflection, like from an apple? We are seeing the 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 are scattered while the lower freqencies are transmitted. At sunset, when the 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
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Special Thanks to A. Graps