## Wednesday, January 6, 2010

### Snell’s Law

The angles of incidence and refraction when light travels from one medium to another can be calculated using Snell’s Law.

Deﬁnition: Snell’s Law
n1sin θ1 = n2 sin θ2
where
n1 = Refractive index of material 1
n2 = Refractive index of material 2
θ1 = Angle of incidence
θ2 = Angle of refraction

Remember that angles of incidence and refraction are measured from the normal, which is an imaginary line perpendicular to the surface.
If
n2 <>1
then from Snell’s Law,
sin θ1 < style="font-style: italic;">θ2.
For angles smaller than 90◦, sin θ increases as θ increases. Therefore,
θ1 < θ2.
This means that the angle of incidence is less than the angle of refraction and the light ray is away toward the normal.
Similarly,if
n2 > n1
then from Snell’s Law,
sin θ1 > sin θ2.
For angles smaller than 90◦, sin θ increases as θ increases. Therefore,
θ1 > θ2.
This means that the angle of incidence is greater than the angle of refraction and the light ray is bent toward the normal.

What happens to a ray that lies along the normal line?

Worked Example : Using Snell’s Law

Question:
A light ray with an angle of incidence of 35◦ passes from water to air.
Find the angle of refraction using Snell’s Law . Discuss the meaning of your answer.
(the refractive index is 1,333 for water and about 1 for air)