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Question #43573

Difficulty: 8/10 | 🟢 Free

The Question

Two coherent monochromatic electromagnetic waves, Wave 1 and Wave 2, propagate in a vacuum along the +z+z direction. Their electric field vectors are given by: E⃗1=E0cos⁡(kz−ωt)i^{\vec{E}_1 = E_0 \cos(kz - \omega t) \hat{i}} E⃗2=E0cos⁡(kz−ωt+π3)j^{\vec{E}_2 = E_0 \cos\left(kz - \omega t + \frac{\pi}{3}\right) \hat{j}} where E0E_0 is a constant amplitude in V/m\text{V/m}, kk is the wave number in rad/m\text{rad/m}, and ω\omega is the angular frequency in rad/s\text{rad/s}. The time-averaged intensity of Wave 1 alone is I0I_0 (in W/m2\text{W/m}^2). The two waves are superposed, and the resultant wave is incident normally on an ideal linear polarizer. The transmission axis of the polarizer lies in the xyxy-plane and makes an angle of 45∘45^\circ with the positive xx-axis. What is the time-averaged intensity of the transmitted wave?

Options

  • A.

    32I0\frac{3}{2} I_0

  • B.

    I0I_0

  • C.

    2I02 I_0

  • D.

    54I0\frac{5}{4} I_0

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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

Superposition Principle|Transverse Nature of EM Waves|Photon Polarization and Selection|Vector Cross-Products

Why This Question Is Tricky

Common Mistakes

Students often fall into the trap of treating the two orthogonal waves as independent, incoherent sources, incorrectly applying Malus's Law to their intensities separately: I=I1cos⁡2(45∘)+I2cos⁡2(45∘)=I0I = I_1 \cos^2(45^\circ) + I_2 \cos^2(45^\circ) = I_0.

This scalar addition completely ignores the superposition principle for coherent waves.

Because the waves are coherent, their electric field vectors must be superposed first, creating an elliptically polarized state.

Projecting this resultant vector onto the analyzer's axis reveals an interference term 2I1I2cos⁡δ2\sqrt{I_1 I_2}\cos\delta that dictates the true transmitted intensity.

This question tests the critical transition from classical scalar wave addition to vector state evolution in polarization.

Quick Facts About This Question

Question Information

SubjectPhysics
ChapterWave Optics - Class 12
ConceptSuperposition Principle
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree