Refractive index Sections 36, 37 and 38

Refraction: When a wave changes direction as it passes from one medium to another

Refraction occurs because the speed of a wave changes as it moves from one medium to another. This change in speed causes the wave to swing round and move in another direction Lenses uses refraction to produce (focus) images of objects

Refraction depends on both materials (medium) and the direction the wave is travelling

Snell’s law Medium 1 Medium 2

In summary the refractive index of medium 2 relative to medium 1 is:

There are two types of refraction: Absolute and Relative For light: The Absolute refractive index of a particular medium is when the first medium is a vacuum or air. Note: Since refractive index is the ratio of speeds the absolute refractive index of a vacuum or air is 1

For absolute refractive index the first medium is a vacuum/air so the first speed is the speed of light in a vacuum (c = 3x10 8 m s -1 ) The equations can be written: Where   is the absolute refractive index of medium 2 and V 2 is the speed of the wave in medium 2 The bigger the value of the absolute refractive index the more optically dense it is and the slower the speed of the wave.

Relative refractive index is when the first medium is not a vacuum/air. The equations are written:

Relative refractive index is when the first medium is not a vacuum or air The relative refractive index can be written in terms of the absolutes of each: Note: when medium 1 is a vacuum or air  1 = 1 and the relative refractive index is written:

Critical angle When a wave goes from more dense to less dense the angle of incidence, i, is smaller than the angle of refraction, r. Therefore, as the angle of incidence is increased the angle of refraction will reach 90 o first When r = 90 o the angle of incidence is called the critical angle (i = c) From Snell’s law: Where:

For angle r to be bigger than angle i medium 1 is more dense than medium 2 When r = 90 o Rewritten: When the second medium is a vacuum or air,  2 = 1 and  1 is the absolute refractive index of medium 1

Total internal reflection For total internal reflection to occur: The wave has to travel from more dense (low speed) to less dense (high speed) AND the angle of incidence is bigger than the critical angle Optical fibres make use of total internal reflection to transmit data inside fibres made from glass