free physics video tutorials for all





Total Internal Reflection


critical angle

90o deviation

180o deviation

optical fibres




Critical Angle


The Critical Angle (co) is the angle of incidence in a dense medium, such that the angle of refraction in the less dense medium is 90o .


Looking at the diagrams(below), as the angle of incidence in the dense medium is increased, the angle of refraction increases towards 90o.


During this time a weak reflected ray, inside the dense medium, is also observed.



critical angle - diagram #1



Only when the angle of incidence in the medium exceeds the Critical Angle does all the light become reflected internally.



critical angle - diagram #2



critical angle - diagram #3



We can formulate an equation for the critical angle using Snell's Law for two media of refractive index n1 & n2 .


refractive index - equation #7


refractive index - equation #8


When θ1= 90o and θ2= co ,


critical angle - equation #1


but sin(90o) = 1, therefore:

critical angle - equation #2


critical angle -equation #5



From work on relative refractive index,


critical angle - equation #3



critical angle - equation #4




back to top



90o Deviation with a 90o prism


Total internal reflection in glass depends on the fact that its critical angle is approximately 42o .


A light ray with an angle of incidence greater than this will be totally internally reflected.


So a light ray with an angle of incidence of 45o will be reflected, and its angle of reflection will also be 45o.


Hence the light ray is deviated through an angle of 90o .



90 deg. deviation from a 90 deg. prism



For 90o deviation, a 90o isosceles glass prism is used. In this way the incident ray and the reflected rays are normal to the surfaces they enter and leave(and therefore unhindered).


Total internal reflection occurs on the hypotenuse.



back to top



180o Deviation with a 90o prism


Besides a 90o deviation, an isosceles prism is also used to produce a 180o deviation, but this time reflection occurs on the equal, adjacent sides of the prism and not on the hypotenuse.



180 deg. deviation from a 90 deg. prism



By tracing two light rays though the prism, it can be seen that the image produced is inverted. As a result of the image being reflected twice, lateral inversion is cancelled.



back to top



Optical Fibres


Optical fibre typically consists of a core of high refractive index surrounded by cladding with a lower refractive index.


Light is totally internally reflected down the fibre at the boundary of the two media.



optical fibre



Optical fibre has a number of advantages over copper wire:



1. less attenuation

2. cheaper metre for metre

3. can carry more information

4. immune to electrical interference

5. safer - no fire risk as with electric currents

6. wire-tapping more difficult





back to top




this week's promoted video


 from Physics Trek



creative commons license

All downloads are covered by a Creative Commons License.
These are free to download and to share with others provided credit is shown.
Files cannot be altered in any way.
Under no circumstances is content to be used for commercial gain.





©copyright 2016 - All Rights Reserved