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Foundation8 past questions

Diffraction of light

Understand the bending of light around obstacles, single slit diffraction patterns, their width, and the effect of wavelength on diffraction.

Why this shows up in the exam

NEET tests your grasp of how light spreads and forms patterns when passing through narrow openings.

How NEET tests this

Numerical · 4 QsStatement analysis · 2 QsDirect recall · 2 Qs

Learn the idea

Diffraction is the spreading of a light wave when it meets an obstacle or a slit whose size is comparable to its wavelength; the pattern you see is the result of interference of the secondary wavelets.

🧠 Memory hook: When the aperture gets *A*pproximately the size of the *λ*ambda, light *A*rranges itself into a fan – remember **A‑λ‑A** (Aperture‑lambda‑spreads).

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

Formulas & facts to keep ready

  • Diffraction: bending/spreading of light around an obstacle or through a narrow opening (NCERT)
  • For a single slit of width a, minima occur at a sinθ = mλ, m=±1,±2,…
  • Angular width of the central maximum ≈2λ/a (in radians)
  • Diffraction is appreciable only when the size of the opening ≈ λ
  • The pattern is explained by Huygens‑Fresnel principle – each point of the wavefront acts as a source of secondary wavelets

How to approach it

  1. 1Read the question and decide whether it asks for a definition, a condition for minima/maxima, or a qualitative trend
  2. 2Recall the exact NCERT statement or formula that matches the demand
  3. 3If numbers are involved, plug them into the appropriate relation (a sinθ = mλ or width≈2λ/a)
  4. 4Eliminate wrong options by checking if they violate the definition or the condition

Worked example — watch it click

Diffraction and interference of light suggest

  • A)Nature of light is electromagnetic.
  • ✅Wave nature.
  • C)Nature is quantum.
  • D)Nature of light is transverse.

The concept behind this problem

The example asks what the simultaneous occurrence of diffraction and interference tells us; both effects can only be explained if light behaves as a wave, so the question tests the link between these phenomena and the wave nature of light.

Step by step

  1. 1Diffraction and interference are phenomena that can only be explained by the wave nature of light.
  2. 2These effects demonstrate superposition of waves and cannot be explained by particle theory alone.
  3. 3While light is electromagnetic and transverse, the question specifically asks what diffraction and interference suggest, which is fundamentally the wave nature.

Watch out

Students often pick “light is electromagnetic” instead of the required answer “wave nature” because they overlook that the question is about the implication of diffraction and interference, not the broader classification of light.

Common slip-ups that cost marks

  • •Confusing diffraction (single‑slit spreading) with interference of multiple slits
  • •Mixing up the condition for minima (a sinθ = mλ) with that for maxima (a sinθ = (m+½)λ)
  • •Choosing “electromagnetic” because light is an EM wave, while the question asks what the phenomena *suggest*

🌟 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 8NEET 1999

The penetration of light into the region of geometrical shadow is called

Push further

More challenging

15 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 15

Monochromatic light of wavelength 550 nm falls on a single slit of width 0.05 mm. A diffraction pattern is observed on a screen placed 1.2 m away from the slit. Calculate the distance of the first dark fringe from the central maximum.

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