Refaction and Refection

Reflection

Reflection is the abrupt change in the direction of propagation of a wave that strikes the boundary between two different media.  At least some part of the incoming wave remains in the same medium.  Assume the incoming light ray makes an angle θ_i with the normal of a plane tangent to the boundary.  Then the reflected ray makes an angle θ_r with this normal and lies in the same plane as the incident ray and the normal.

Law of reflection: θ_i = θ_r

Refraction.-

Refraction is the change in direction of propagation of a wave when the wave passes from one medium into another, and changes its speed.  Light waves are refracted when crossing the boundary from one transparent medium into another because the speed of light is different in different media.  Assume that light waves encounter the plane surface of a piece of glass after traveling initially through air as shown in the figure to the right.

If f is the frequency of the wave and T = 1/f is the period, i.e. the time interval between successive crests passing a fixed point in space, then 

λ₁ = v₁T = cT/n₁ and λ₂ = v₂T = cT/n₂, or λ₁/λ₂ = n₂/n₁.