Light-reflection & refraction?

Asked by abhinavsinha | 25th May, 2010, 09:56: PM

Expert Answer:

Dear student,

the stars don’t really twinkle. Rather, this is an effect of our atmosphere. In space, the stars would appear as steady pinpoints. Turbulence in the atmosphere is largely responsible for the twinkling effect, so the steadier the atmosphere, the less the stars seem to twinkle and sparkle, and the more unsteady the atmosphere, the more the stars seem to twinkle and sparkle.

The stars are very far away. In fact, they are so far away that they basically are like point sources as far as we are concerned. That means that the light appears to come from a single point in the sky (unlike the Moon in which light comes from a disk about half a degree across, or a planet in which light appears to come from a very small disk in the sky). As this light passes through Earth’s atmosphere, it is bent a bit whenever it passes through a region of slightly denser air. Different colors of light, in principle, take slightly different paths, since the degree of refraction (bending) that light undergoes is frequency dependent. So, the path of the light through the air from the star to your eye takes a sort of convoluted path.

Now, if this were all that was going on, then the star would simply appear slightly out of position in the sky rather than twinkling. If the different colors were bent far enough apart, then the star might appear as a tiny little splotch of colors (like a tiny rainbow) rather than a pinpoint. That would be if the air were completely unmoving. But, that isn’t what happens. The air moves around. Solar heating causes convection. Winds blow. Different parts of the air are compressed and rarefied as the air moves. These different regions refract light differently (which is part of the reason that the light takes a convoluted path through the atmosphere, anyway).



Answered by  | 26th May, 2010, 12:15: PM

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