CBSE Class 10 Science 2019 · Paper 31/3/1 · Question 13(a)

Atmospheric refraction is the refraction (bending) of light by the Earth's atmosphere, caused by the variation in the optical density (refractive index) of air: the air near the Earth is denser and the air higher up is rarer. Starlight entering the atmosphere undergoes continuous refraction before it reaches the Earth: passing into denser and denser air, it keeps bending towards the normal. The eye sees the star along the direction in which the light finally enters it, so the apparent position of the star is slightly different from its actual position - slightly higher above the horizon. Labelled diagram: the Earth and an observer; the atmosphere as layers with refractive index increasing towards the Earth; the curved path of starlight from the true position of the star to the eye; and the dashed line of sight traced back to the apparent position of the star, above the true position.