Q.1 what is refraction? why this happen ?
Answer. When a Ray of light going from one medium and enters in another medium than it debates from its path this phenomenon is called refraction of light. The main cause of refraction is the change in velocity of light and velocity of light changes when it travels from one medium to another medium.
Question 2 What is refractive index ?
Answer It is the ratio of the speed of light into different medium it is a constant and unitless refractive index (u)= speed of light in first medium / speed of light in second medium (u)= v1/v2