Q.Explain polarisation of light.
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An ordinary transverse light wave has its electric-field vibrations spread over every direction in the plane perpendicular to the direction it travels; this is called unpolarised light. If those vibrations are restricted so they occur only along a single fixed direction in that perpendicular plane, the light is called plane polarised (or simply polarised) light; if the vibration amplitude varies with direction (largest along one axis, non-zero but smaller along the perpendicular axis) rather than being restricted entirely to one direction, the light is called partially polarised. The plane containing the actual field vibrations is the plane of vibration, and the plane perpendicular to it (containing the ray direction) is the plane of polarisation. Polarisation is the general phenomenon of restricting a transverse wave's vibrations to one particular direction; since interference and diffraction occur for both transverse and longitudinal waves, but polarisation is only possible for transverse waves, the existence of polarisation in light is the definitive proof that light waves are transverse, not longitudinal. Unpolarised light can be converted into plane polarised light by four distinct techniques: selective absorption (dichroic materials such as polaroid sheets, which transmit vibrations along one axis and absorb the rest), reflection (at the Brewster/polarising angle), double refraction (in crystals such as calcite, via a Nicol prism) and scattering (by small particles such as air molecules). In selective absorption, the first polaroid used to produce plane polarised light from an unpolarised beam is called the polariser, and a second polaroid used afterward to te …
Polarisation describes what happens to the vibrations of light's electric field when unpolarised light is passed through a device such as a polaroid. …
Polarisation confines the vibrations of light's electric vector to one plane. Unpolarised light vibrates in all transverse directions; a polaroid transmits only one, producing plane-polarised light — evidence that light is a transverse wave.
Explanation.
Light is a transverse electromagnetic wave, in which the electric field E vibrates perpendicular to the direction of propagation. In ordinary (unpolarised) light coming from a source such as the sun or a lamp, these E-vibrations occur equally in all directions in the plane perpendicular to the direction of travel.
When such light is passed through a polaroid (or reflected/scattered under suitable conditions), only the component of E parallel to the polaroid's transmission axis is allowed through; vibrations in other directions are absorbed. The emergent light then has its E-vector confined to one single plane and is called plane-polarised light.
Thus polarisation is the process of restricting the vibrations of light to a single plane.
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Showing the 12 most recent of 20 on this concept.
- CBSE 2026Set A1 markMCQQ.Natural light from the sun is (A) polarised (B) unpolarised (C) partially polarised (D) linearly polarised
›Reveal solutionSolution
Natural sunlight vibrates equally in all planes perpendicular to its direction of travel, so it is unpolarised.
Light is a transverse electromagnetic wave, so its electric field oscillates in the plane perpendicular to the direction of propagation. In light from the Sun the emission comes from countless independent atoms, each radiating with a random orientation of the electric vector. Averaged over these, there is no preferre …
- CBSE 2025Set X11 markQ.Polarization of light shows that light is a ________ wave.
›Reveal solutionSolution
transverse Only transverse waves, whose vibrations are perpendicular to the direction of propagation, can be polarized. The fact that light can be polarized proves that light is a transverse w …
- CBSE 2025Set D1 markMCQQ.In polarised light the angle between plane of vibration and plane of polarization is (A) 0° (B) 45° (C) 90° (D) 180°
›Reveal solutionSolution
By definition the plane of polarization is at right angles to the plane of vibration, so the angle is 90°.
In polarised light the electric-field vector vibrates in one plane, called the plane of vibration. The plane of polarization is conventionally defined as the plane perpendicular to the plane of vibration (containing the directio …
- CBSE 2025Set D1 markMCQQ.Which of the following cannot be polarized? (A) Sound waves (B) Light waves (C) Radio waves (D) X-rays
›Reveal solutionSolution
Only transverse waves can be polarized; sound in air is longitudinal, so it cannot be polarized.
Polarization requires the vibrations to be perpendicular to the direction of propagation, i.e. the wave must be transverse. Light waves, radio waves and X-rays are all electromagnetic (transverse) waves and can be polarized.
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- CBSE 2025Set A1 markQ.Write True or False: For an unpolarised wave the displacement remains constant with respect to time.
›Reveal solutionSolution
The statement is False: unpolarised light has a rapidly and randomly varying direction of vibration, not a constant one.
Ordinary (unpolarised) light consists of a large number of wave trains emitted by different atoms, each vibrating in its own transverse plane. The resultant electric field vector's direction of vibration therefore changes randomly and rapidly with time, and on average is symmetric about the direction of propagation — this is precisely why it is called 'unpolarised'. In contrast, a polarised wave has its vi …
- CBSE 2025Set ANNUAL1 markMCQQ.The confirmation of transverse nature of light waves is by(a) reflection(b) refraction(c) polarisation(d) interference
›Reveal solutionSolution
Polarisation restricts vibrations to a single plane, which is only possible for a transverse wave; longitudinal waves cannot be polarised at all — so the fact that light CAN be polarised proves it is transverse.
Reflection, refraction, and interference occur for both transverse and longitudinal waves (e.g. sound, a longitudinal wave, also reflects, refracts, and interferes), so these phenomena alone cannot distinguish the two. Polarisation, however, is possible only for transverse waves, since it involves restricting the oscillation direction (which is …
- CBSE 2025Set ANNUAL1 markMCQQ.The transverse nature of light is shown in :(a) scattering(b) interference(c) polarisation(d) diffraction
›Reveal solutionSolution
Polarisation is possible only for transverse waves, so light's ability to be polarised demonstrates its transverse character.
Working
Scattering, interference and diffraction occur for both transverse and longitudinal waves, so they do not by themselves reveal a wave's transverse nature. Polarisation, however, involves restricting oscillation to a single plane perpendicular to the direction of travel -- something only a transverse wave's oscillating field can undergo. That light can be polarised …
- CBSE 2024Set ANNUAL1 markMCQQ.Natural light from the sun is -(a) polarised(b) unpolarised(c) partially polarised(d) linear polarised
›Reveal solutionSolution
Ordinary light from the sun consists of a huge number of independent wave trains vibrating randomly in every direction perpendicular to the direction of travel, so it is unpolarised.
Light is a transverse wave, so its electric field vibrates in a plane perpendicular to the direction of propagation. In light emitted directly by a natural source like the sun, this vibration direction changes randomly and rapidly, with no preferred plane — such light is called unpolarised.
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- CBSE 2024Set ANNUAL1 markMCQQ.The transverse nature of light is shown in :(a) scattering(b) interference(c) polarisation(d) diffraction
›Reveal solutionSolution
Polarisation is a property unique to transverse waves, so the fact that light can be polarised is direct evidence of its transverse nature.
Working
Scattering, interference and diffraction are exhibited by both transverse and longitudinal waves (e.g. sound, a longitudinal wave, also shows interference and diffraction), so these phenomena alone do not distinguish a transverse wave from a longitudinal one.
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- CBSE 2023Set F1 markMCQQ.Light is a transverse wave because it shows (A) Reflection (B) Polarization (C) Interference (D) Diffraction
›Reveal solutionSolution
Polarization is possible only for transverse waves ⇒ it proves light is transverse.
Reflection, interference and diffraction are shown by BOTH longitudinal and transverse waves, so they cannot distinguish the two. Polarization — restricting the vibrations to a single plane — is possible only when the oscillations are perpendicular to t …
- CBSE 2023Set F1 markMCQQ.Which is used to produce polarised light? (A) Prism of flint glass (B) NaCl prism (C) Nicol prism (D) Biprism
›Reveal solutionSolution
A Nicol prism produces plane-polarised light.
A Nicol prism is made from a calcite crystal cut and cemented with Canada balsam. Inside the crystal an unpolarised beam splits into two plane-polarised rays (ordinary and extraordinary) by double refraction. The ordinary ray undergoes total internal reflection at the balsam layer and is absorbed, while the extraordinary ray passe …
- CBSE 2022Set ANNUAL1 markQ.Light waves are ____ in nature. (transverse, longitudinal)
›Reveal solutionSolution
Light is a transverse wave — its electric and magnetic field oscillations are perpendicular to the direction of propagation.
In a transverse wave, the disturbance (here, the oscillating electric and magnetic fields) is perpendicular to the direction the wave travels; in a longitudinal wave, the disturbance is along the direction of travel (like sound). Light, being an electromagnetic wave, has its E and B fields oscillating perpendicular to each other and to the direction of propagation, which makes it transverse. This transverse nature is confirmed experimentally by …
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