0.1 Geometrical optics - grade 10  (Page 6/14)

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

Snell's Law

$$n_{1}sin{\theta }_1=n_{2}sin{\theta }_2$$

where

$n_1$ =	Refractive index of material 1
$n_2$ =	Refractive index of material 2
${\theta }_1$ =	Angle of incidence
${\theta }_2$ =	Angle of refraction

Remember that angles of incidence and refraction are measured from the normal, which is an imaginary line perpendicular to the surface.

Suppose we have two media with refractive indices $n_1$ and $n_2$ . A light ray is incident on the surface between these materials with an angle of incidence ${\theta }_1$ . The refracted ray that passes through the second medium will have an angle of refraction ${\theta }_2$ .

If

then from Snell's Law,

For angles smaller than ${90}^{\circ }$ , $sin\theta$ increases as $\theta$ increases. Therefore,

This means that the angle of incidence is greater than the angle of refraction and the light ray is bent toward the normal.

Similarly, if

then from Snell's Law,

For angles smaller than ${90}^{\circ }$ , $sin\theta$ increases as $\theta$ increases. Therefore,

This means that the angle of incidence is less than the angle of refraction and the light ray is away toward the normal.

Both these situations can be seen in [link] .

What happens to a ray that lies along the normal line? In this case, the angle of incidence is $0^{\circ }$ and

This shows that if the light ray is incident at $0^{\circ }$ , then the angle of refraction is also $0^{\circ }$ . The ray passes through the surface unchanged, i.e. no refraction occurs.

Investigation : snell's law 1

The angles of incidence and refraction were measured in five unknown media and recorded in the table below. Use your knowledge about Snell's Law to identify each of the unknown media A - E. Use [link] to help you.