Refraction is the process in which light will bend away or towards the normal at the point of incidence when light comes from one optical medium to another optical medium.
During sunset, sun is still visible after it has dropped below the horizon.It is so because the light coming from sun is refracted when it passes through the atmosphere.
Our atmosphere has not a constant density.It varies depending its position from earth surface. Hence, refractive index of each layer of atmosphere varies which results in the refraction of sun light for which sun is visible though it is geometrically below the horizon.
Hence, the correct answer to the question is refraction.
