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Foundation4 past questions

Total internal reflection and critical angle

Learn the conditions for total internal reflection, the concept of critical angle, and their applications in optics.

Why this shows up in the exam

Questions frequently test your ability to identify and calculate when total internal reflection occurs in different media.

How NEET tests this

Numerical · 3 QsDirect recall

Learn the idea

Total internal reflection occurs only when light travels from a medium of higher refractive index to a lower one and the incidence angle exceeds the critical angle; at the critical angle the refracted ray just grazes the interface, i.e., makes 90° with the normal.

🧠 Memory hook: Critical angle = the ray’s ‘skate‑board’ that just glides along the surface – it’s 90° to the normal.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • TIR condition: n₁>n₂ and θᵢ>θc
  • Critical angle: sinθc = n₂/n₁ (where n₁ is the incident medium)
  • Using speeds: sinθc = v₁/v₂ because n = c/v
  • At θ = θc the refracted ray is parallel to the boundary, so θr = 90°
  • If n₁≤n₂, total internal reflection cannot occur

How to approach it

  1. 1Identify the incident and transmitted media and write their refractive indices (or speeds)
  2. 2Check that the light is going from the higher‑index (slower) to the lower‑index (faster) medium
  3. 3Compute θc from sinθc = n₂/n₁ (or sinθc = v₁/v₂)
  4. 4Compare the given angle of incidence with θc; if equal, answer 90° for the refracted angle; if larger, TIR occurs

Worked example — watch it click

In total internal reflection when the angle of incidence is equal to the critical angle for the pair of media in contact, what will be angle of refraction?

  • A)0°
  • B)equal to angle of incidence
  • ✅90°
  • D)180°

The concept behind this problem

The example tests the key fact that at the critical angle the refracted ray runs parallel to the interface, so its angle with the normal is exactly 90°.

Step by step

  1. 1At critical angle, refracted ray grazes the interface, making angle of refraction = 90°.

Watch out

Students often answer that the refracted angle equals the incidence angle or is zero, forgetting the grazing‑ray condition.

Common slip-ups that cost marks

  • •Mixing up n₁ and n₂ in the sine formula
  • •Assuming TIR can happen when light goes from rarer to denser medium
  • •Thinking the refracted angle becomes 0° at the critical angle instead of 90°

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 4NEET 2019

In total internal reflection when the angle of incidence is equal to the critical angle for the pair of media in contact, what will be angle of refraction?

Push further

More challenging

6 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 6

Light travels from air (refractive index 1.0) into a diamond (refractive index 2.42). What is the critical angle for this interface?

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