Physics Q27 – NEET-UG 2025

An unpolarized light beam travelling in air is incident on a medium of refractive index

1.73

at Brewster's angle. Then-

Clue 1 of 3: Brewster's Law

When unpolarized light is incident at Brewster's angle, the reflected light is completely plane-polarized.

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● Video Walkthrough

Step 1: Apply Brewster's Law and calculate Brewster's angle✦ Active

Brewster's law states that when unpolarized light is incident at Brewster's angle ( $i_{p}$ ), the reflected light is completely plane-polarized. The angle is given by $\tan i_{p} = n$ , where $n$ is the refractive index of the medium relative to air ( $n_{1} = 1$ for air, $n_{2} = 1.73$ for the medium).

\tan i_{p} = 1.73

Calculating the angle: $i_{p} = \arctan (1.73)$ . Since $\tan (60^{\circ}) \approx 1.732$ , we have $i_{p} \approx 60^{\circ}$ .

Step 2: Determine the nature of reflected and transmitted light and angles○ Expand

At Brewster's angle, the reflected light is completely plane-polarized. According to the law of reflection, the angle of reflection ( $r$ ) is equal to the angle of incidence ( $i_{p}$ ). Therefore, $r \approx 60^{\circ}$ .

The transmitted light at Brewster's angle is partially polarized.

Also, at Brewster's angle, the reflected and refracted rays are perpendicular, which means $i_{p} + r^{'} = 90^{\circ}$ , where $r^{'}$ is the angle of refraction.

60^{\circ} + r^{'} = 90^{\circ} ⟹ r^{'} = 30^{\circ}

💡 Teacher's Secret Hint

Remember that only the reflected light is completely polarized at Brewster's angle; the transmitted light is partially polarized.

Step 3: Evaluate the options○ Expand

Based on our findings:

Option (1) states that reflected light is completely polarized and the angle of reflection is close to $60^{\circ}$ . This is consistent with our calculations.

Option (2) is incorrect because reflected light is completely polarized, not partially, and the angle of reflection is $60^{\circ}$ , not $30^{\circ}$ .

Option (3) is incorrect because transmitted light is partially polarized, not perfectly polarized.

Option (4) is incorrect because transmitted light is partially polarized, not completely polarized.

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Physics Question 27 – NEET-UG 2025

Clue 1 of 3: Brewster's Law

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