in
the denser medium, the angle r in the rarer medium is equal to 90°,
when the ray of light will just graze the surface between the media.
For any angle greater than i, i.e. I the light ray cannot
emerge and is reflected back within the denser medium. This is known as
the phenomenon of total reflection. All rays striking on MM' with the incident angle greater than i will suffer total reflection.
The value of the critical angle is expressed by the formula
sin
where n = index of refraction, and i = critical
angle. The higher the value of n, the smaller is the
