When light travels from an optically denser medium to a rarer medium, why does the critical angle

Option 4 : glass to water

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Concept:

Total Internal Reflection (TIR): 

• When a ray of light goes from denser to the rarer medium it bends away from the normal and as the angle of incidence in denser medium increases, the angle of refraction in the rarer medium also increases and at a certain angle, the angle of refraction becomes 90°, this angle of incidence is called critical angle (C).
• When the Angle of incidence exceeds the critical angle then light ray comes back into the same medium after reflection from the interface.
• This phenomenon is called Total internal reflection (TIR).

Explanation:

• TIR happens when a ray of light travels from an optically denser medium to a rarer medium. 
• Glass is optically denser than water. 
• The speed of light in glass is less than that of water. 
• When light travels from glass to water, TIR may happen. 
• Glass to water is the correct option. 

Considering other options

• Air to water: It is not possible as air is optically rarer than water. 
• Air to glass:  air is optically rarer than glass.
• Water to glass: water is optically rarer than glass. 

Snell's law:

• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for the light of a given colour and for the given pair of media.

​\(⇒ \frac{sin\, i}{sin\, r}=Constant\)

Where I = angle of incidence and r = angle of refraction

Refractive index:

• When light travels from medium 1 to medium 2, then the ratio of the speed of light in medium 2 to the speed of light in medium 1 is called the refractive index of medium 1 with respect to medium 2.

\(⇒ μ_{12}=\frac{v_2}{v_1}\)

• If medium 1 is denser compared to medium 2, then the refractive index of medium 1 with respect to medium 2 will be greater than 1.
• If medium 1 is rarer compared to medium 2, then the refractive index of medium 1 with respect to medium 2 will be less than 1.

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