For example, since the speed of light in water is v = 2.25 × 10^8 m/s, the index of
refraction of water is
Notice that n has no units; it’s also never less than 1.
The equation that relates θ 1 and θ 2 involves the index of refraction of the incident
medium (n 1 ) and the index of refraction of the refracting medium (n 2 ); it’s called
Snell’s law (know this for the test).
n 1 sinθ 1 = n 2 sinθ 2If n 2 > n 1 (i.e., when the light slows down in n 2 ), then Snell’s law tells us that θ 2 <
θ 1 ; that is, the beam will bend (refract) toward the normal as it enters the medium.
On the contrary, if n 2 < n 1 (i.e., when the light speeds up in n 2 ), then θ 2 > θ 1 , and
the beam will bend away from the normal.
- A beam of light in air is incident upon a piece of glass, striking
the surface at an angle of 60°. If the index of refraction of the glass
is 1.5, what are the angles of reflection and refraction?
Here’s How to Crack It
If the light beam makes an angle of 60° with the surface, then it makes an angle of
30° with the normal; this is the angle of incidence. By the law of reflection, then,
the angle of reflection is also 30°. We use Snell’s law to find the angle of
refraction. The index of refraction of air is close to 1, so we can say that n = 1 for
air.