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Q & A: Sunlight and its components

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Most recent answer: 04/26/2013
The sunlight reaches us as white light and not as it's components .why?
- Seba (age 19)

Hi Seba,

The light from the sun does in fact reach us as its different components. If you used a spectrum analyzer (like a prism) to split the light into its constituents, you would see a whole range of colors. You don't see them all separately because they overlap in space and time, and your eye "mixes" them all together so that you see a yellow-ish or white-ish light.

In what cases would we see the components of sunlight individually? Well, I'm sure you've seen a rainbow... this effect occurs when refraction in water droplets separates the different colors out in space, and our eyes can resolve them one at a time.

What about separation in time? Just for fun, I dreamed up a (somewhat ridiculous) imaginary situation in which the sun's light would be different colors at different times.

When light travels in a medium (like glass), different colors sometimes travel at different speeds. This effect, called dispersion, is usually quite small, and doesn't happen in vacuum (like space). Now, let's get crazy and pretend that space has a lot of dispersion, so that blue light travels much slower than red light. Now, the sun is continuously illuminating the earth, so the different colors would still overlap when they hit our eyes; just as a bit of blue light hits our eyes, a bit of red light which was emitted earlier would also arrive, and we would see white.

However, during a lunar eclipse in this imaginary universe, we might have a chance to see the colors arriving at different times. Just as the sun reappears from behind the moon, rays of light traveling at the usual speed of light would take ~1 second to reach us. If blue light was traveling slower, it would arrive later. So, the first glimpse of light would be red, but would quickly turn to white again.


David Schmid


(published on 04/26/2013)

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