Critical Angles


Concept of Critical angles

Generally, the refractive index of a denser transparent substance is higher than that of a less dense material, which would mean that light would travel slower in the substance. A general rule is that when a ray travels from a medium to another with a higher refractive index, it'll bend towards the normal. When a ray travels from a medium to another with a lower refractive index, it'll bend away from the normal.

Due to this fact, as the angle of incidence of a ray increases in a medium with a higher refractive index, the ray passing through to a medium with lower refractive index will bend away from the normal until it meets 90 deg. with the normal, and on the boundary between the two mediums, which is then referred to as the critical angle. 

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