Thursday, May 20, 2010

When the sun rises can we say that red light reaches first than the violet?

The velocity of red light is greater than that of violet.

When the sun rises can we say that red light reaches first than the violet?
The problem is, the air refracts the light as the sun rises. When you see the sun as it rises, it is still actually below the horizon, and it is just that the layers of air are bending the light down so you can see it. The color that gets bent (refracted) the most is the violet light, then the blue, then the green and so on. That is what is responsible for the green flash (since your eye is not as sensitive to the blue and the violet colors, it sees green first). The red actually arrives last, because of these refraction effects.

The speed of light difference between red and violet is extremely small in air, so this should not have a large influence in the timing, particularly when the atmosphere is only a few tens of miles thick at most (99% of the atmosphere is in the lowest 30 Km).

After thinking about this a minute, I realized that you are asking because the sun looks red when it first comes up. This is NOT because the red gets here first. The blue wavelengths are scattered away from the almost white sunlight by the nitrogen and oxygen in the air, leaving the red color. As the sun rises, it goes through less air, and less blue is scattered away, and the sun becomes orange. As the sun rises even higher in the sky, less blue is removed, so the sun looks yellow.

Its due to the fact that Violet's wave length is shorter than Red's.

Oxygen and Nitrogen scatters the violet and blue light due to their short wave length. During the sun set the light have to travel a greater disitance and as a result a large amount of blue and violet light has been scattered leaving only the longer orange and red wave lenghts.
Reply:Errr... for the most part, red light and violet light travel at the same speed. There is, however, a *tiny* effect from refraction that causes red light to move at a slightly faster speed in a medium than violet. So yes, after travelling 8 minutes from the sun to the earth, red light might reach us just a few milliseconds faster.

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