1) Refraction is the chnage of direction of light as it passes from one medium to another.
2) A lens forms an image by refracting light.
3) In a convex or converging lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lense to the principle focus is called the focal length.
Refractive index= Sini/ Sinr
Where i is the angle of incidence and r is the angle of refraction