Total internal reflection

and critical angle

 




 

1) Light when passes from a denser medium to an optically lighter medium or rarer medium, light deviates from the normal at the point of incident



2) when light incidents at Q it deviates along the QQ' as shown in the figure.



3) When incident angle increases a certain angle achieved when the refracted light just passes along the interface of the two medium .


This angle is called the critical angle denoted as C. as shown in the diagram.




4) If you increase the incident angle more than the critical angle, light never goes to the rarer medium or lighter medium. The light completely returns back to the original medium, as you can see SS' light path.




5) This is like a reflection. As there is no enters to the rarer medium, it is a complete reflection, so this is called total internal reflection.



6) Now you can see when the incident angle is the critical angle C, the angle of refraction is



  thus 











7) From this we can calculate the critical angle if we know the refractive index of the medium.



8) For water & air system, refractive index of water w.r.t. air is 1.33, so





Therefore when light goes from water to air then, refractive index of air w.r.t. water is

















Therefore critical angle of water w.r.t. air is  



Some examples and applications of total internal reflection are

optical fibers,  diamond,  Mirage.