Critical angle is affected by the refractive index of second medium with respect to the first medium. This in turn depends on:
Wavelength: Critical angle increases with increase in wavelength (least for violet).
Temperature: Critical angle increases with increase in temperature.
When the angle of incidence in water reaches a certain critical value, the refracted ray lies along the boundary, having an angle of refraction of 90-degrees. This angle of incidence is known as the critical angle; it is the largest angle of incidence for which refraction can still occur. For any angle of incidence greater than the critical angle, light will undergo total internal reflection.
Definition: The angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally reflected.
Total internal reflection (TIR)
Introduction: When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium. This reflection is called the internal reflection.
Definition: Total internal reflection is defined as the complete reflection of a light ray at the boundary of two media, when the ray is in the medium with greater refractive index.
Relation between refractive index and critical angle
Let us consider medium 1 − Incident medium and 2 − Refractive medium, Then value of critical angle can be derived by Snell's law. n1sinθi=n2sinθr n1sinθcrit=n2sin90∘ sinθcrit=n1n2 sinθcrit=n121
n12 : Refractive index of denser medium 1 with respect to rarer medium 2.