Atmospheric refraction is the refraction of light caused by the earth's atmosphere due to change in the refractive indices of different layers of the atmosphere.
1. Twinkling of stars:
The phenomenon that causes stars to twinkle is atmospheric refraction. That is, the air above us is a big ocean of not very dense fluid that has a small bending effect on light that passes through it. If it were perfectly still, the light from a star would follow a path that stays put, and the star's image would appear stable.
But the atmosphere is always moving; winds in every level of it cause slight density differences to move around, and different densities bend light by different amounts. And so the light rays from a star zig and zag as they head toward your eyes. That makes them appear to twinkle. The reason this happens for stars and not planets or other larger, "extended" objects, though, is that a star's apparent (angular) size is so very, very tiny. It is what is called in optics, a "point source."
When there's a little bit more of an apparent disk (these are measured in angle, and we're talking in the arc-second range and smaller), then it takes a lot more bending of the incoming light for the rays coming from all the parts of that disk to get bent away from your direction so that the (extended) object would appear to "blink" momentarily, and so, appear to twinkle. And it doesn't take much of an apparent disk to prevent twinkling.
But it also depends on what's happening in the air above you at any particular time, too. On some clear nights, there will be a lot of roiling in the atmosphere, and stars will twinkle more. Other clear nights, when the upper air is calm, twinkling will all but stop.
The whole effect is sometimes likened to what you see on the bottom of a swimming pool in bright sunlight. If the surface of the water is pretty wavy, there will be a lot of moving light and dark patterns on that pool floor. If nobody's been in the water for a while, and there's no wind, the surface gets smooth, and the moving patterns disappear.
2. Advanced sun rise and delayed sun set:
The layers of air nearer to earth are denser than those above it. At sunrise and sunset when the sun is below the horizon, the light rays starting from the sun are incident on these layers. They pass through successively denser layers and thus get bent more and more towards the normal until they fall upon the eye of the observer O. To the observer O these rays appear to come from S' which is above the horizon. It is for this reason that the sun is visible to us a little before it rises above the horizon and so also till a little later it sets below the horizon. The difference of time is about 2 minutes each for an early rise and late setting of the Sun.
