Polarisation

topicmedium9 MCQ

Plane polarised light, Malus's law and Brewster's law. (Physics › Wave Optics, NEET UG syllabus.)

What is Polarisation?

The phenomenon of restricting the vibrations of a transverse wave (like light) to a single plane.

Key formula / rule: Malus's Law

Define unpolarised and plane polarised light.
Explain the phenomenon of polarisation and its significance.
Describe different methods of producing polarised light, focusing on reflection and selective absorption.
State and apply Brewster's Law to calculate Brewster's angle or refractive index.

Confusing the plane of polarisation with the plane of vibration.

Term: Polarisation
Meaning: The phenomenon of restricting the vibrations of a transverse wave (like light) to a single plane.

Term: Unpolarised Light
Meaning: Light in which the electric field vibrations occur in all possible planes perpendicular to the direction of propagation.

Term: Plane Polarised Light
Meaning: Light in which the electric field vibrations are confined to a single plane perpendicular to the direction of propagation.

Term: Polariser
Meaning: A device or material (e.g., Polaroid) that produces plane polarised light from unpolarised light.

Term: Analyser
Meaning: A polarising device used to detect and measure the ° of polarisation of light, or to determine the plane of polarisation.

Term: Brewster's Angle (Polarising Angle)
Meaning: The angle of incidence for which the reflected light is completely plane polarised, and the reflected and refracted rays are perpendicular to each other.

Define unpolarised and plane polarised light.
Explain the phenomenon of polarisation and its significance.
Describe different methods of producing polarised light, focusing on reflection and selective absorption.
State and apply Brewster's Law to calculate Brewster's angle or refractive index.
State and apply Malus's Law to calculate the intensity of light transmitted through an analyser.
Understand the working principle and applications of Polaroids.

Name: Malus's Law
Note: I is the transmitted intensity, I₀ is the intensity of the plane polarised light incident on the analyser, and θ is the angle between the transmission axis of the analyser and the plane of polarisation of the incident light.
Expression: I = I₀ cos²θ

Name: Brewster's Law
Note: μ is the refractive index of the medium, and i_p is Brewster's angle (or polarising angle) at which the reflected light is completely plane polarised.
Expression: μ = tan i_p

Confusing the plane of polarisation with the plane of vibration.
Incorrectly applying Malus's Law (e.g., using sin²θ instead of cos²θ, or using I₀ as unpolarised intensity directly).
Not understanding that reflected light at Brewster's angle is completely polarised, but refracted light is only partially polarised.
Forgetting that when unpolarised light passes through a polariser, the intensity becomes I₀/2, not I₀.
Misidentifying the angle θ in Malus's Law (it's between the transmission axis of the analyser and the plane of polarisation of incident light, not necessarily the polariser).

When two polaroids are oriented such that their pass axes are perpendicular to each other, what is the intensity of light transmitted through the second polaroid if unpolarised light of intensity I₀ is incident on the fi…
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Unpolarised light of intensity 200 W/m² is incident on a polaroid. The transmitted light then passes through a second polaroid whose pass axis makes an angle of 60 degrees with that of the first polaroid. What is the int…
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Which of the following statements is INCORRECT regarding polarisation by reflection?…
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