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Question #43122

Difficulty: 8/10 | 🟢 Free

The Question

A plane electromagnetic wave of angular frequency ω\omega is incident from vacuum onto the flat surface of a uniaxial anisotropic crystal. The boundary is the xx-yy plane (z=0z=0), and the crystal occupies the region z>0z > 0. The plane of incidence is the xx-zz plane, and the angle of incidence is θi=60∘\theta_i = 60^\circ. The optic axis of the crystal lies along the xx-axis. The ordinary and extraordinary refractive indices of the crystal are no=3n_o = \sqrt{3} and ne=2n_e = 2, respectively. If the incident wave is linearly polarized such that it excites only the extraordinary wave in the crystal, what is the angle of refraction θr\theta_r of the ray (the direction of energy flow) with respect to the zz-axis inside the crystal?

Options

  • A.

    tan⁡−1(23)\tan^{-1}\left(\frac{2}{3}\right)

  • B.

    tan⁡−1(12)\tan^{-1}\left(\frac{1}{2}\right)

  • C.

    sin⁡−1(34)\sin^{-1}\left(\frac{\sqrt{3}}{4}\right)

  • D.

    30∘30^\circ

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Intuition_Breaker

Train your brain to resist "obvious" answers that are designed to deceive your intuition.

Concepts Behind This Question

Snell’s Law Vector Form|Boundary Condition Reflection|Vector Analysis

Why This Question Is Tricky

Common Mistakes

This question tests the distinction between the wave vector (phase velocity direction) and the ray vector (group velocity/energy flow direction) in anisotropic media.

Students commonly fall into the trap of applying the scalar Snell's law (n1sin⁡θ1=n2sin⁡θ2n_1 \sin\theta_1 = n_2 \sin\theta_2) using either non_o or nen_e, which incorrectly assumes the ray and wave vectors are collinear.

The correct physical framework requires using the vector form of Snell's law to enforce the continuity of the tangential wave vector, and subsequently applying the gradient of the dispersion relation (v⃗g=∇kω\vec{v}_g = \nabla_k \omega) to determine the true direction of energy propagation.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterRay Optics and Optical Instruments - Class 12
ConceptSnell’s Law Vector Form
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree