A ray of laser of a wavelength 630 nm is incident at an angle of 30° at the diamond-air interface. It is going from diamond to air. The refractive index of diamond is 2.42 and that of air is 1. Choose the correct option.

Refractive index (n) is a dimensionless quantity that has no units. Since both numerator and denominator are in meters per seconds (m/s), both units cancel each other. This makes refractive index simply a number. 

Absolute refractive index is the ratio of speed of light in air/vacuum to speed of light in any medium. It is closely related to refractive index, except that it is relative to vacuum and not to any other medium. This is different from relative refractive index because it is the ratio of speed of light in one medium compared to the speed of light in another medium.

This type of refraction is the bending of light rays when they pass through the atmosphere of Earth because of the change in the air density. The density of air varies with pressure and temperature. 

Due to the atmospheric refraction, celestial objects appear higher than they actually are. This leads to an advanced sunrise and a delayed sunset as well as the twinkling of stars.

When light from space enters the Earth's atmosphere, it slows down and changes direction as light moves from a rarer to a denser medium, which causes refraction.

The refractive index is an optical property of the material that describes how much does the light bend or refract while passing from one medium to another. It is the ratio of spee of light in vacuum (c) to speed of light in a material (v)$n=\frac{c}{v}$ Refractive index is the reason which causes refraction. When light travels from a rarer medium (low refractive index) to denser medium (higher refractive index), it will slow down and bend toward the normal. Refractive index also changes according to the wavelength of light. Air has 1.003 refractive index, water has 1.33 refractive index and glass has a refractive index of 1.5 to 1.9.