The atmospheric refraction is responsible for the twinkling of stars. As the light from the stars enters the earth's atmosphere it suffers refraction and causes the twinkling of light.
Refraction- When a light ray travels from one transparent medium to another transparent medium then it deviates from its original path and this phenomenon is called Refraction.
For example- when light travels from the Rarer medium (Air) to the Denser medium (Water) is deviates towards the Normal.
Laws of refraction
The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.
The Normal, Incident ray, Refracted ray lie on the same plane at the point of Incidence.