When unpolarised light hits a surface (like glass or water) at one special angle, the reflected ray becomes completely plane-polarised. This special angle is called Brewster's angle (i_B), and it follows Brewster's law: tan i_B = mu (the refractive index of the surface). Memory hook: at Brewster's angle the reflected and refracted rays are exactly 90 degrees apart, so "reflected is fully polarised, refracted is only partly polarised."
At Brewster's angle i_B the reflected ray (fully polarised) and the refracted ray (partly polarised) are exactly 90 degrees apart, and tan i_B = mu.
Your doubts, answered
At Brewster's angle, is the reflected light or the refracted light completely polarised?
The reflected light is completely (100%) polarised. The refracted (transmitted) light is only partially polarised because most of the light energy passes into the medium and it still carries both vibration directions. This exact point was asked in NEET 2024. Simple way to remember: only the weak reflected beam is fully cleaned up, the strong transmitted beam is not.
In which direction does the reflected light vibrate at Brewster's angle?
The electric vector of the reflected light vibrates perpendicular to the plane of incidence (parallel to the reflecting surface). The plane of incidence is the plane containing the incident ray and the normal. So the reflected E-field is perpendicular to that plane. NEET 2018 tested this exact wording.
What is the relation tan i_B = mu and where does it come from?
Brewster's law says tan i_B = mu, where mu is the refractive index of the second medium (with respect to the first). It comes from combining two facts: at Brewster's angle the reflected and refracted rays are perpendicular, and Snell's law mu = sin i / sin r. Putting r = 90 - i gives sin r = cos i, so mu = sin i / cos i = tan i. Full derivation is on the brewster-angle-formula page.
What is the angle between the reflected ray and the refracted ray at Brewster's angle?
Exactly 90 degrees. At Brewster's angle the reflected ray and the refracted ray are perpendicular to each other. This is the key geometric clue: whenever a question says reflected and refracted rays are perpendicular, the angle of incidence is Brewster's angle and tan i = mu.
Why must Brewster's angle be between 45 and 90 degrees for glass or water?
Because tan i_B = mu and every real transparent medium (glass, water) has mu greater than 1. If mu > 1 then tan i_B > 1, which means i_B > 45 degrees. Since an angle of incidence is always less than 90 degrees, i_B lies between 45 and 90 degrees. NEET 2020 asked exactly this.
⚠️ The NEET trap ✗ At Brewster's angle both the reflected and the refracted (transmitted) light become completely polarised. ✓ Only the reflected light is completely polarised. The refracted/transmitted light is only partially polarised. 🧠 One beam only. The weak reflected ray is 100% polarised; the strong transmitted ray keeps both components, so it is partly polarised. NEET 2024 and 2025 both trap students here.
Real NEET questions
NEET 2018
Unpolarised light is incident from air on a plane surface of a material of refractive index mu. At a particular angle of incidence i, it is found that the reflected and refracted rays are perpendicular to each other. Which option is correct?
A · i = sin^-1(1/mu)
B · Reflected light is polarised with its electric vector perpendicular to the plane of incidence ✓
C · Reflected light is polarised with its electric vector parallel to the plane of incidence
D · i = tan^-1(1/mu)
Solution: Step 1: Reflected and refracted rays perpendicular means i is Brewster's angle. Step 2: Then tan i = mu, so i = tan^-1(mu), not tan^-1(1/mu) or sin^-1(1/mu). This rules out options A and D. Step 3: At Brewster's angle the reflected beam is completely polarised, and its electric vector vibrates perpendicular to the plane of incidence. Correct answer: B.
NEET 2024
An unpolarized light beam strikes a glass surface at Brewster's angle. Then
A · The refracted light will be completely polarised
B · Both the reflected and refracted light will be completely polarised
C · The reflected light will be completely polarized but the refracted light will be partially polarized ✓
D · The reflected light will be partially polarized
Solution: Step 1: At Brewster's angle the reflected beam is completely plane-polarised. Step 2: The refracted (transmitted) beam still contains both vibration directions, so it is only partially polarised. Step 3: So the reflected light is fully polarised and the refracted light is partly polarised. Correct answer: C.
NEET 2025
An unpolarized light beam travelling in air is incident on a medium of refractive index 1.73 at Brewster's angle. Then
A · Both reflected and transmitted light are perfectly polarized with angles of reflection and refraction close to 60 and 30 degrees
B · Transmitted light is completely polarized with angle of refraction close to 30 degrees
C · Reflected light is completely polarized and the angle of reflection is close to 60 degrees ✓
D · Reflected light is partially polarized and the angle of reflection is close to 30 degrees
Solution: Step 1: Apply Brewster's law: tan i_B = mu = 1.73 = square root of 3. Step 2: tan i_B = 1.73 gives i_B = 60 degrees. Step 3: By the law of reflection, angle of reflection = angle of incidence = 60 degrees, and the reflected light is completely polarised. Step 4: The transmitted light is only partially polarised, so A and B are wrong. Correct answer: C.
Solved Wave Optics NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Brewster's law is tan i_B = mu, where i_B is Brewster's angle (the polarising angle) and mu is the refractive index of the reflecting medium relative to the medium of incidence.
Is light polarised by reflection always fully polarised?
No. Light is fully (completely) polarised on reflection only when the angle of incidence equals Brewster's angle. At any other angle the reflected light is only partially polarised.
What is Brewster's angle for glass of mu = 1.5?
tan i_B = 1.5, so i_B = tan^-1(1.5) which is about 56.3 degrees. For water (mu = 1.33) it is about 53 degrees.
Why is the refracted ray only partially polarised at Brewster's angle?
Most of the incident energy passes into the medium as refracted light, and it carries both vibration components (only the perpendicular component is preferentially reflected away), so the transmitted beam stays partially polarised.
How is Brewster's angle related to the angle of refraction?
At Brewster's angle the reflected and refracted rays are perpendicular, so i_B + r = 90 degrees. This means the angle of refraction r = 90 - i_B.
