Question 1

What is total internal reflection?

Accepted Answer

Total internal reflection (TIR) is the complete reflection of light back into a denser medium when it strikes the boundary with a rarer medium at an incidence angle greater than the critical angle. No light is refracted out — 100% of the energy is reflected.

Question 2

What is the critical angle?

Accepted Answer

The critical angle C is the incidence angle in the denser medium for which the angle of refraction in the rarer medium is exactly 90°. It satisfies sin C = μ₂/μ₁, which becomes sin C = 1/μ when light travels from a medium of refractive index μ into air.

Question 3

Why does a diamond sparkle so much?

Accepted Answer

Diamond has a very high refractive index (μ = 2.42), giving a very small critical angle of about 24.4°. Almost every ray inside the stone meets a facet at more than 24.4°, so it undergoes repeated total internal reflections before finally exiting through the top — producing its characteristic brilliance.

Question 4

How do optical fibres use total internal reflection?

Accepted Answer

An optical fibre has a dense glass core surrounded by a rarer cladding. Light entering the fibre strikes the core–cladding boundary at angles greater than the critical angle, so it undergoes continuous total internal reflection and stays trapped inside, travelling long distances with almost no loss.

Question 5

Can total internal reflection happen when light goes from air into glass?

Accepted Answer

No. TIR requires light to travel from a denser to a rarer medium. Going from air (rarer) into glass (denser), the refracted ray bends toward the normal and the refraction angle is always less than 90°, so a critical angle does not even exist in that direction.

Total Internal Reflection & the Critical Angle — Watch the Refracted Ray Die

Practice

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