1.      How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?

When ray of light enters a prism, it bends because of refraction of light. When the ray of light finally emerges out of the prism. it deviates drastically from its original path. This happens because of unique shape of prism.

Different colours in the visible spectrum have different speeds. Due to this, different colours bend at different angles of deviation. As a result, the emergent light appears as a band of seven colours: the colours which are the components of white light. These colours are Violet. Indigo, Blue, Green, Yellow, Orange and Red.

Segregation of white light into its different components is called dispersion of light.