CONCEPT: - Refraction: The phenomena by which a ray of light bends its path when it travels from one transparent medium to another is called refraction of light. - The bending happens because the speed of light varies in different mediums. - Refractive index: The ratio of the speed of light in the air by the speed of light in the medium is called the refractive index. More will be the ratio of refractive index between two mediums more will be the bending of light. - Atmospheric Refraction: The different layers of the atmosphere have a different refractive index. The ray of light coming from some outer space changes continuously bends its path while passing through different atmospheric layers. EXPLANATION: - The atmosphere of the earth is made of different layers. - It is affected by winds, varying temperatures, and different densities as well. - When light from a distant source (a star) passes through our turbulent (moving air) atmosphere, it undergoes refraction many times. - When we finally perceive this light from a star, it appears to be twinkling. - This is because some of the light rays reach us directly and some bends away from and toward us. It happens so fast that it gives a twinkling effect.