Atmospheric refraction and scattering

Look through the hot air just above a fire or a road on a summer day: things behind it seem to waver. Hot air is lighter and bends light a little differently from the cooler air around it, and these layers keep moving. Bending of light by the air is atmospheric refraction.

Why stars twinkle. Starlight passes through layer after layer of air, which keeps shifting. So the star's light keeps bending a little differently: its apparent position jumps about and its brightness flickers. A star is so far away that it is just a point of light.

Why planets don't twinkle. A planet is much closer, so it looks like a tiny disc: many points of light, not one. The flickers from all these points average out, and the planet shines steadily.

Because of atmospheric refraction, we see the Sun about 2 minutes before it actually rises above the horizon and about 2 minutes after it has actually set. The Sun's disc also looks slightly flattened at sunrise and sunset.

Scattering. A beam of sunlight entering a dusty or smoky room through a small hole becomes visible: tiny particles scatter the light towards you. So does sunlight streaming through the trees in a misty forest. This is the Tyndall effect.

"Why is the sky blue?" The molecules of air are very small, and very small particles scatter blue light much more than red light. Scattered blue light reaches your eyes from all over the sky. An astronaut high above the air sees a dark sky: with no air, there is nothing to scatter the light.

Red is scattered the least, so red light travels through fog and smoke best. That is why danger signals are red: they can be seen from far away.