Refraction of Light

1
Refraction of Light
2

• Light travels in straight lines. However, when
  light travels through different mediums, its path
  is deviated. This deviation is called
  refraction.
• Each medium has its own level of refraction.
• We can calculate the index of refraction, n, of a
  medium using
• n c/v
• Where c is the speed of an electromagnetic wave
  in a vacuum, 3.00 x 108 m/s
• and v is the speed of light in that medium in m/s

3

• Example
• Using the table of values below, identify the
  medium through which light travels when its
  velocity is 8.57 x 107 m/s.
• n c/v
• c 3.00 x 108 m/s n c/v
• v 8.57 x 107 m/s n 3.00 x 108 m/s
• 8.57 x 107 m/s
• n 3.50, gallium phosphide

Medium Water Ethyl alcohol Plexiglass Diamond Gallium phosphide
Index of refraction (n) 1.33 1.36 1.51 2.42 3.50
4

• Note
• The greater the index of refraction of a medium,
  the slower the speed at which light will travel
  through it.

5

• A relative index of refraction can also be
  calculated when a light ray does not pass through
  a vacuum.
• The relative index of refraction is a ratio of
  the index of refractions of the two mediums
  involved.

6

• Example
• Calculate the relative index of refraction of a
  light ray that travels through water and then
  through Plexiglass.
• nR n2 / n1
• n1 index of refraction of the first medium
• n2 index of refraction of the second medium
• nR 1.51 / 1.33 1.14

7
Geometry of Refraction

• Since light is travelling in two different
  mediums, the angle of incidence is no longer
  equal to the angel of refraction.
• Laws of refraction
• The incident ray, the normal and the refracted
  ray are all on the same plane.

8

• 2. The ratio of the sine of the angle of
  incidence and refraction are equal to the ratio
  between the index of refraction of the two
  mediums.
• sin ?i n2
• sin ?R n1
• n1 sin ?i n2 sin ?R

9

• Example
• A light travel from a diamond (n2.42) into water
  (n1.33). If the rays have an angle of incidence
  of 30o, calculate the angle of refraction.
• n1 sin ?i n2 sin ?R
• 2.42 x sin (30) 1.33 x sin ?R
• ?R 65o

10

• EXAMPLE
• A light ray travelling through air encounters a
  glass cup with an angle of incidence of 35o.
  The light ray then travels through water and
  finally through ice. Determine the final angle
  of refraction once the light ray exits the ice.

11
Total Internal Reflection

• When a light ray travels in a medium with a high
  refractive index and encounters one with a low
  refractive index, refraction does NOT always
  occur the light ray can be reflected instead.
• The angle of incidence will determine whether the
  ray is reflected of refracted.

12

• If the angle of incidence is greater than or
  equal to the critical angle, then internal
  reflection occurs. If not the ray will be
  refracted into the second medium.

13

• We can determine the value of the critical angle
  by using
• ?C sin-1 (n2/n1)
• Where ?C is the critical angle
• n1 is the refractive index of medium 1
• n1 is the refractive index of medium 2

14

• Example
• An aquarium has a built in light source. The
  light rays hit the aquariums glass walls at an
  angle of incidence of 65o.

1. Determine whether reflection or refraction
   occurs.
2. At what angle will the light ray reflect/refract?