The Critical Angle/Total Internal Reflection
The critical angle and total internal reflection are applicable only when light travels
from a medium with a greater index of refraction to a medium with a smaller index of
• critical angle: the angle at which light is refracted at 90º.
• total internal reflection: the reflection of incident light
from a boundary due to the incident light angle exceeding
the critical angle.
Refraction Critical Angle Total Internal Reflection
Light is transmitted to the new The transmitted light The incident light is reflected from
is refracted at 90º.
medium with a new angle. from the boundary. The
law of reflection applies.
n2 θ2 n2 n2
n1 n1 n1
The incident light ray The incident light is at an Any incident angle greater
is less than the critical angle that causes the than the critical angle causes
angle (θ1<θc). transmitted light to refract light to be reflected from the
at 90º. (θ1=θc, θ2=90º). the boundary between the
two media. (θ1>θc, θ1=θ2)
• When the transmitted light refracts at 90º, θ1=θc and
• n1sinθc=n2sin 90º
•Any incident angle greater that the critical is reflected from the boundary of the
•The index of medium 1 must be greater than the index of refraction of medium 2
for the critical angle and total internal reflection to occur.