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# What is Total Internal Reflection or TIR?

## Total Internal Reflection

Its name itself explains the phenomena. Total internal reflection is the complete reflection of a ray of  light within the medium.

When the ray of light  moves from optical denser medium to rarer medium the refracted ray bends away from the normal. But as soon as the angle of incidence is greater than the critical angle, the refracted ray gets reflected back into the same medium. This  phenomenon is known as  “Total Internal Reflection”.

Figure : Total Internal Reflection

For more clarity, look at the figure. Suppose Medium 1 (water) has refractive index n1 and Medium 2 (air) has refractive index n2. When a light moves from medium 1 to medium 2, it Keep on increasing the angle of incidence till the angle of refraction becomes equal to 900.

The angle of incidence for which angle of refraction becomes 900 is  known as “critical angle”.

But as soon as we further increase the angle of incidence (i.e. θ1) of light beam in medium 1 greater than critical angle; instead of refracting, the beam of light gets reflected back in the same medium giving rise to the phenomena of total internal reflection.

Point to keep in mind that when a beam of light moves from a rarer to denser medium, refracted ray bends towards the normal whereas when a beam of light moves from a denser to rarer  medium, refracted ray bends away from the normal.

## Critical Angle

Let us calculate the value of critical angle.

Hence at critical angle θ1= θc and  θ2= 90o .

Using Snell‘s law: n1 sin θ1 = n2 sin θ2

n1 sin θc = n2 sin90o

sin θc = n2 / n1

θc = sin-1 (n2 / n1)

Observed value of critical angle depends on the materials present on each side of the boundary.

## Condition For Total Internal Reflection

These are the important conditions require to obtain the phenomena of total internal reflection.

• The beam of light should travel from denser to rarer medium.
• Angle of incidence of a beam of light should be greater than critical angle.

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