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NEET 2025 Physics Question 27 Solution

Topic: Wave Optics · Official NTA Booklet Code 45

$NEET 2025 Physics Question 27: Brewster’s Law, Angle of Polarization & State of Reflected/Refracted Rays$

Question

An unpolarized light beam travelling in air is incident on a medium of refractive index 1.73 at Brewster's angle. Then

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Verified Against NTA Final Answer Key

Option (1) : reflected light is completely polarized and the angle of reflection is close to 60°

📚 Wave Optics Easy

📋 Given Parameters & Problem Setup

Refractive Index of Medium (μ) 1.73 (≈ √3)

Incident Light Unpolarized light travelling in air

Incident Condition Incident at Brewster’s angle (i_B)

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The 30-Second Exam Hall Shortcut: ⚡ Brewster’s Tan Law & Polarization Rule

⏱️ Save ~40 seconds

tan i_B = μ = 1.73 = √3 ⟹ i_B = 60° ⟹ Angle of reflection = 60° and Reflected ray is 100% COMPLETELY POLARIZED!

1 Apply Brewster’s Law: tan i_B = μ = 1.73 ≈ √3.

2 Angle of incidence: i_B = arctan(√3) = 60°.

3 Law of reflection: Angle of reflection r' = i_B = 60°.

4 Brewster’s Postulate: At Brewster’s angle, the REFLECTED beam is COMPLETELY plane polarized (electric vector perpendicular to plane of incidence).

5 The transmitted (refracted) beam is only PARTIALLY polarized.

6 Match Option (1) directly in under 10 seconds!

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Why You Can 100% Trust This Shortcut: Brewster’s Law of Polarization (NCERT Physics · Wave Optics: μ = tan(i_p)).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Calculate Brewster’s Angle of Incidence

Brewster’s law states that when light is incident at a polarizing angle i_B, the tangent of the polarizing angle is equal to the refractive index μ of the reflecting medium:

\tan i_B = \mu = 1.73 \approx \sqrt{3} \implies i_B = 60^\circ

• By the fundamental law of reflection, angle of reflection equals angle of incidence: r' = i_B = 60°.

Step 2

State of Polarization of Reflected vs. Transmitted Light

At Brewster’s angle, the reflected and refracted rays are mutually perpendicular (i_B + r = 90°). Electrons oscillating parallel to the refracted ray cannot radiate along their axis of oscillation towards the reflected direction.

i_B + r = 90^\circ \implies r = 90^\circ - 60^\circ = 30^\circ

• Therefore, the reflected light contains only vibrations perpendicular to the plane of incidence (s-polarization) and is completely plane polarized.
• The transmitted light contains both parallel and residual perpendicular components, making it only partially polarized.

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360° Option Elimination Engine Key Feature

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4-Option Deep Scan

1 A

✓ 100% Correct

Verdict & Scientific Reason:

100% correct. i_B = 60°, angle of reflection is 60°, and reflected light is completely plane polarized.

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Why this is airtight: None; textbook Brewster’s law.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Reflected light is completely (not partially) polarized, and angle of reflection is 60° (30° is angle of refraction).

⚠️

Student Fallacy (Why candidates pick this trap): Confusing angle of reflection with angle of refraction.

3 C

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Transmitted light is only partially polarized; it is never completely polarized by a single Brewster reflection.

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Student Fallacy (Why candidates pick this trap): Falsely assuming transmitted light is also completely polarized.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Transmitted light is partially polarized.

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Student Fallacy (Why candidates pick this trap): Swapping the polarization state between reflected and transmitted beams.

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NTA Trap Alert: Transmitted Light Polarization Fallacy

A major trap in wave optics: Candidates assume that if reflected light is completely polarized, transmitted light must also be completely polarized. Transmitted light is PARTIALLY polarized because only a fraction of the perpendicular component reflects.

🛡️ How to avoid the -1 mark: Remember: Reflected = 100% Completely polarized; Transmitted = Only Partially polarized!

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Core NEET Syllabus Standard

NEET Physics Core Syllabus · Wave Optics Standard Benchmark

Core Scientific Principle Tested:

"At Brewster’s angle tan i_B = μ, reflected light is completely plane-polarized perpendicular to the plane of incidence."

Syllabus Focus: Polarization by Reflection & Brewster’s Law

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

tan 60° = √3 = 1.73 ⟹ Reflected is 100% Polarized at 60°!

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NEET 2025 Physics Question 27 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Brewster’s Tan Law & Polarization Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Calculate Brewster’s Angle of Incidence

State of Polarization of Reflected vs. Transmitted Light

360° Option Elimination Engine Key Feature

NTA Trap Alert: Transmitted Light Polarization Fallacy

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Wave Optics

Young’s Double Slit Experiment with White Light

State of Polarization of Reflected and Refracted Light at Brewster’s Angle

Dependence of Angular Width on Wavelength and Screen Distance

Brewster's Law and Range of Brewster's Polarizing Angle: 45° < ib < 90°

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