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MCQEasyJEE 2026Wavefront & Huygens Principle

JEE Physics 2026 Question with Solution

Given below are two statements:

Statement I: A plane wave after passing through a prism remains a plane wave, but passing through a small pin hole may become a spherical wave.

Statement II: The curvature of a spherical wave emerging from a slit will increase for increasing slit width.

In the light of the above statements, choose the correct answer:

  • A

    Both Statement I and Statement II are false

  • B

    Both Statement I and Statement II are true

  • C

    Statement I is false but Statement II is true

  • D

    Statement I is true but Statement II is false

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: Two statements about the nature of wavefronts after passing through a prism, pin hole, and slit.

Find: Which statement is true and which statement is false.

Concept: Wavefronts describe the shape of propagating waves. Plane waves have flat wavefronts, while spherical waves have curved wavefronts.

Step 1: Analyze Statement I. A prism only refracts light and changes its direction, not the nature of the wavefront. Thus, a plane wave remains a plane wave after passing through a prism.

A small pin hole acts like a point source due to diffraction, causing the emerging wavefront to become spherical.

⇒Statement I is true.\Rightarrow \text{Statement I is true.}

Step 2: Analyze Statement II. For a slit:

  • Smaller slit width ⇒\Rightarrow stronger diffraction
  • Stronger diffraction ⇒\Rightarrow more curvature of wavefront

Thus, as slit width increases, diffraction decreases and curvature decreases, not increases.

⇒Statement II is false.\Rightarrow \text{Statement II is false.}

Step 3: Final conclusion.

Statement I is true, Statement II is false.\text{Statement I is true, Statement II is false.}

Therefore, the correct option is D.

Common mistakes

  • Assuming that a prism changes a plane wave into a spherical wave. This is wrong because a prism mainly changes the direction of propagation by refraction, not the basic planar nature of the wavefront. Instead, distinguish between refraction by an extended optical element and diffraction by a very small aperture.

  • Thinking that increasing slit width increases diffraction and hence increases curvature. This is wrong because diffraction becomes weaker when the slit width increases. Instead, remember that narrower apertures produce greater spreading and more curved emerging wavefronts.

  • Confusing a pin hole with an ordinary refracting element. This is wrong because a sufficiently small pin hole behaves like an approximate point source due to diffraction. Instead, connect small aperture size with spherical wavefront formation.

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