Refraction – Learn


Refraction is the change in direction of a wave caused by a change in its speed. Waves change speed when they pass from one medium to another. This could be a light wave passing from air into glass or a water wave moving from deep water into shallow water. Waves travel slower in denser mediums.

During refraction the velocity of a wave changes but the frequency stays the same. Therefore, according to v=f\lambda , iff remains constant, than the wavelength must also change. The impact of a change in medium on the speed of a wave and the wavelength can be illustrated with a diagram showing a series of wavefronts moving from one medium to another, parallel to the boundary:

Refraction can be illustrated with a similar diagram where the wavefronts are not parallel to the boundary:

Diagrams of refraction usually illustrate the refraction with a single ray travelling perpendicular to the wavefronts as shown in the next section.


The direction that the wave refracts (bends) is dependent on the change in medium:

  • If the wave moves from a less dense to a more dense medium the wave will refract toward the normal (Diagram A).
  • If the wave moves from a more dense to a less dense medium the wave will refract away from the normal (Diagram B).

This can be illustrated both ways by considering a light ray travelling between air (low density) and glass (high density):


Absolute Refractive Index

The absolute refractive index of a medium is the ratio of the velocity of the wave in a vacuum to that of its velocity in the medium.

Mathematically: Absolute refractive index =n=\cfrac { speed\; of\; wave\; in\; vacuum }{ speed\; of\; wave\; in\; medium } =\cfrac { c }{ v } =\cfrac { 3\times { 10 }^{ 8 } }{ v }

Denser medium have a larger refractive index. Light in a vacuum has a refractive index of 1. Generally we apply a refractive index of 1 to light travelling through air.

The greater the difference in absolute refractive index, the more the light ray will refract.

Check out the simulation at the bottom of this page


Example 1:

A wave travels from medium A into medium B as shown below. The wave refracts toward the normal.

a) How does the frequency of the wave compare between medium A and B?

b) How does the speed of the wave compare between medium A and B?

c) How does the wavelength of the wave compare between medium A and B?

d) How do the absolute refractive indexes of medium A and B compare?

Answers:

a) Same. Frequency is always a constant for a wave travelling through different medium.

b) The wave travels slower in medium B.

c) The wavelength is shorter in medium B.

d) Medium A has a lower refractive index compared to B. This is because the light ray bends toward the normal and this occurs when a light ray travels from a less dense to a more dense medium.


Example 2:

Three mediums A, B and C have absolute refractive indexes 2.4, 1.3 and 1.7 respectively. Draw two diagrams to illustrate the direction a light ray travels in:

a) moving from A into B:

b) moving from C into A:

Answers:

a) 

b) 


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