Huygens' principle treats every point on a wavefront as a source of secondary spherical wavelets that spread out in the forward direction with the same speed as the wave itself; the new wavefront at any later instant is the common tangent (the forward envelope) to all these secondary wavelets. A wavefront is the locus of all points that a wave disturbance reaches at the same instant, and i
Wave Optics
Class 129 previous-year questions from this chapter every option and the correct answer, free · where the marks are
1. Huygens' principle
Exam focus: Huygens' wave theory correctly predicted that light slows down in a denser medium, v = c/n — the opposite of Newton's corpuscular prediction — a difference that later measurement of light's speed in water settled in the wave theory's favour.
Wave Optics: Introduction and Huygens' Principle
Physics Wallah - Alakh Pandey
Dedicated lecture introducing wave optics and Huygens' principle/wavefronts specifically.
2. Interference and Young's double slit
Exam focus: Bright and dark fringes in Young's double slit are evenly and identically spaced, β = λD/d, not narrower at the dark fringes — and the whole pattern needs coherent sources, which is why two ordinary, independent lamps never show a stable pattern.
Young's Double Slit Experiment and Interference
Physics Wallah - Alakh Pandey
Full dedicated lecture on YDSE and interference of light waves.
PhysicsNEET 2024show ▾If the monochromatic source in Young's double slit experiment is replaced by white light, then:
- A.There will be a central dark fringe surrounded by a few coloured fringes
- B.There will be a central bright white fringe surrounded by a few coloured fringes✓
- C.All bright fringes will be of equal width
- D.Interference pattern will disappear
Solution
With white light every wavelength produces a bright fringe at the centre (zero path difference), so the central fringe is white; on either side the different colours spread at different fringe widths and only a few coloured fringes are seen before they wash out.
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Save for spaced revision (free account)PhysicsNEET 2023show ▾For Young's double slit experiment, two statements are given below:
Statement I: If the screen is moved away from the plane of the slits, the angular separation of the fringes remains constant.
Statement II: If the monochromatic source is replaced by another monochromatic source of larger wavelength, the angular separation of the fringes decreases.
In the light of the above statements, choose the correct answer from the options given below:
- A.Both Statement I and Statement II are true.
- B.Both Statement I and Statement II are false.
- C.Statement I is true but Statement II is false.✓
- D.Statement I is false but Statement II is true.
The worked solution is part of a paid pack.
PhysicsNEET 2022show ▾In a Young's double slit experiment, a student observes 8 fringes in a certain segment of the screen when monochromatic light of 600 nm wavelength is used. If the wavelength of light is changed to 400 nm, then the number of fringes he would observe in the same region of the screen is:
PhysicsNEET 2020show ▾In Young's double slit experiment, if the separation between coherent sources is halved and the distance of the screen from the coherent sources is doubled, then the fringe width becomes:
- A.Double
- B.Half
- C.Four times✓
- D.One-fourth
The worked solution is part of a paid pack.
PhysicsNEET 2019show ▾In a double-slit experiment, when light of wavelength 400 nm was used, the angular width of the first minimum formed on a screen placed 1 m away was found to be . What will be the angular width of the first minimum if the entire experimental apparatus is immersed in water ()?
3. Diffraction and polarisation
Exam focus: Polarisation only happens for transverse waves, and the fact that light can be polarised is itself the proof light is transverse. In single-slit diffraction, a sinθ = nλ locates the minima, not the maxima — the reverse of double-slit's condition for maxima.
Wave Optics — Full Chapter (incl. Diffraction and Polarisation)
NCERT Wallah
No single dedicated video covering exactly diffraction plus polarisation together from a preferred channel could be verified (the main series splits them into separate lectures), so this full wave-optics one-shot is used, which includes both alongside earlier topics.
PhysicsNEET 2025show ▾An unpolarised light beam travelling in air is incident on a medium of refractive index 1.73 at Brewster's angle. Then:
- A.The reflected light is completely polarised and the angle of reflection is close to 60°✓
- B.The reflected light is partially polarised and the angle of reflection is close to 30°
- C.Both reflected and transmitted light are perfectly polarised with angles of reflection and refraction close to 60° and 30°, respectively
- D.The transmitted light is completely polarised with an angle of refraction close to 30°
Solution
Brewster's angle: . The reflected beam (at ) is completely polarised; the refracted beam, at , is only partially polarised.
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Save for spaced revision (free account)PhysicsNEET 2025show ▾The intensity of transmitted light when a polaroid sheet is placed between two crossed polaroids at from the polarisation axis of one of the polaroids is ( is the intensity of polarised light after passing through the first polaroid):
- A.
- B.
- C.✓
- D.
Solution
First polaroid → . The middle sheet at : . The last, crossed with the first, is at to the middle sheet: .
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Save for spaced revision (free account)PhysicsNEET 2024show ▾An unpolarised 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 polarised but the refracted light will be partially polarised.✓
- D.The reflected light will be partially polarised.
Solution
At Brewster's angle the reflected light is completely plane-polarised (electric vector perpendicular to the plane of incidence), while the refracted beam contains both components and is only partially polarised.
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