Q.Assertion (A): A light wave can be considered to travel from one point to another along a straight line.
Reason (R): Wavelength of light is very small compared to size of ordinary objects in daily life.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Huygens' Principle and Wavefronts
A wavefront is the locus of all points that are in the same phase (state) of vibration at a given instant; because points on a wavefront are all in step, a wavefront is always perpendicular to the direction of the wave's propagation (the ray). A point source at a finite distance produces spherical wavefronts, a line (extended) source produces cylindrical wavefronts, and any source located effectively at infinity produces plane wavefronts. Huygens' principle is a geometrical construction rule stating that every point on a wavefront acts as a source of secondary spherical wavelets, spreading outward with the wave's own speed; the new wavefront at a later time t is the common forward tangent (the 'envelope') of all these secondary wavelets, each having grown to radius ct from its originating point. This construction correctly reproduces spherical wavefronts from a nearby point source and plane wavefronts from a distant source, and it explains how a wavefront propagates without needing to track individual rays. Its one acknowledged shortcoming is that it …
Light bends noticeably around an obstacle only when the obstacle is about as small as its wavelength, and since everyday objects are vastly larger than light's tiny wavelength, diffraction is negligible and light appears to travel straight, so the reas …
Ray (straight-line) propagation of light is a good approximation exactly because light's wavelength is far smaller than the objects/apertures it interacts with in everyday life.
Assertion (A) is true: over everyday distances and apertures, light can be treated as travelling in straight lines (rays) — this is the basis of ray/geometrical optics. Reason (R) is also true: the wavelength of visible light (~400–700 nm) is indeed extremely small compared to the size of ordinary objects and apertures encountered in daily life. R correctly explains A: diffraction effects (bending of waves around obstacles/edges) become significant only when the size of an obstacle or aperture is com …
Showing the 12 most recent of 26 on this concept.
- CBSE 2026Set V11 markMCQQ.According to Huygen's principle, speed of the secondary wavelets is :(a) twice that of the wave(b) zero(c) same as that of the wave(d) infinite
›Reveal solutionSolution
(c) same as that of the wave …
- CBSE 2026Set A1 markMCQQ.The wavefront due to a point source at a finite distance from the source is (A) plane (B) circular (C) cylindrical (D) spherical
›Reveal solutionSolution
A point source radiates uniformly outward, giving spherical wavefronts (they become nearly plane only very far away).
A wavefront is the surface joining points of the same phase. A point source sends out disturbances equally in every direction, so at any finite distance the set of in-phase points forms a sphere centred on the source — a ** …
- CBSE 2026Set ANNUAL1 markMCQQ.The phase difference between any two points on a wavefront is(a) 0°(b) 45°(c) 90°(d) 120°
›Reveal solutionSolution
A wavefront is defined as the locus of points that are in the same phase of oscillation, so the phase difference between any two points on it is zero.
By definition, a wavefront is a surface of constant phase - every point on it has travelled the same optical path length from the source and is therefore oscillating in exactly the same …
- CBSE 2026Set ANNUAL1 markMCQQ.The concept of secondary wavelets was given by(a) Fresnel(b) Newton(c) Huygens(d) Maxwell
›Reveal solutionSolution
Huygens' principle says every point of a wavefront is itself a source of secondary wavelets, and the new wavefront is their common tangent surface.
Christiaan Huygens proposed that every point on an existing wavefront can be regarded as a source of new, secondary spherical wavelets that spread out in the forward direction with the speed of the wave in that medium. The surface that is tangential to all these secondary wavelets at any later instant gives the new position of the wavefront. This principle (Huygens' principle) is used to explain reflection, refraction, and diffraction of light p …
- CBSE 2026Set ANNUAL1 markMCQQ.The phase difference between two points located on the same wavefront is:(a) 2π(b) 0(c) π(d) π/2
›Reveal solutionSolution
A wavefront is, by definition, the locus of points that are all at the same stage (phase) of their vibration, so the phase difference between any two of its points is zero.
A wavefront is defined as the continuous locus of all points in a medium that are oscillating in the same phase, having been disturbed by the wave at the same instant relative to the source (e.g. a sphere for a point source, or a plane far from the source). Since every point on a wavefront is, by construction, at the identical stage of its oscillation cycle, the phase difference between any two such points …
- CBSE 2026Set ANNUAL1 markQ.Draw the diagram of wave front produced by a point source of light.
›Reveal solutionSolution
Figure — Explicit 'Draw the diagram of wave front produced by a point source' hard gate. Catalog fig-10-1 is exactly a A point source produces spherical wavefronts.
A point source of light emits waves equally in all directions, so at any given instant, all the points at the same distance from the source are vibrating in the same phase. The locus of these points — the wavefront — is therefore a sphere centred on the source, and as time progresses, this spherical wavefront expands outward with radius increasing at t …
- CBSE 2025Set ANNUAL1 markMCQQ.Assertion (A): A light wave can be considered to travel from one point to another along a straight line. Reason (R): Wavelength of light is very small compared to size of ordinary objects in daily life.(a) Both A and R are correct and R is the correct explanation of A.(b) Both A and R are correct but R is not correct explanation of A.(c) A is correct but R is incorrect.(d) A and R both are incorrect.
›Reveal solutionSolution
Ray (straight-line) propagation of light is a good approximation exactly because light's wavelength is far smaller than the objects/apertures it interacts with in everyday life.
Assertion (A) is true: over everyday distances and apertures, light can be treated as travelling in straight lines (rays) — this is the basis of ray/geometrical optics. Reason (R) is also true: the wavelength of visible light (~400–700 nm) is indeed extremely small compared to the size of ordinary objects and apertures encountered in daily life. R correctly explains A: diffraction effects (bending of waves around obstacles/edges) become significant only when the size of an obstacle or aperture is com …
- CBSE 2025Set ANNUAL1 markQ.What is the shape of the wave front when light is diverging from a point source?
›Reveal solutionSolution
A point source emits waves equally in all directions, so surfaces of constant phase are spheres centred on the source.
A wavefront is the locus of all points that a wave disturbance reaches at the same instant (a surface of constant phase). A point source radiates light uniformly in all directions in three-dimensional space, so at any given instant, all points at the same distance from the source have travelled for the same time and are in the same phase. The locus of such points is a sphere centred at the source. …
- CBSE 2025Set ANNUAL1 markMCQQ.According to Huygens, light waves are(a) Mechanical, longitudinal(b) Mechanical, transverse(c) Electromagnetic(d) Mechanical, spherical
›Reveal solutionSolution
Huygens proposed light as a wave phenomenon needing a material medium (the luminiferous ether) to travel through, and modelled it, like sound, as a longitudinal mechanical wave.
Huygens' wave theory explained reflection, refraction and the (then-mysterious) phenomenon of double refraction using the idea of wavefronts built from secondary wavelets. However, since it treated light as a mechanical disturbance in a medium (analogous to sound waves in air), the vibrations were taken to be longitudinal.
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- CBSE 2025Set ANNUAL1 markMCQQ.Hugen's wave theory allows us to know about :(a) the wavelength of the wave.(b) the velocity of the wave.(c) the amplitude of the wave.(d) the propagation of wavefronts.
›Reveal solutionSolution
Huygens' principle is a geometrical construction that predicts how a wavefront advances with time, i.e., the propagation of wavefronts.
Huygens' wave theory states that every point on a wavefront acts as a source of secondary wavelets, which spread out in all directions with the wave's speed. The new wavefront, at a later time, is the envelope (tangent surface) of these secondary wavelets. This construction is a geometric tool specifically for tracking how a wavefront propagates and changes shape …
- CBSE 2024Set A11 markMCQQ.According to Huygen's principle, the speed of the secondary wavelets is(a) twice that of the wave(b) zero(c) same as the wave(d) infinite
›Reveal solutionSolution
(c) same as the wave …
- CBSE 2024Set ANNUAL1 markMCQQ.According to Huygen's principle, the direction of energy propagation of a wave is -(a) along the surface of wave front(b) perpendicular to wave front(c) at an angle of 45∘ to wave front(d) None of these
›Reveal solutionSolution
Energy flows along the ray direction, and rays are always normal to the wavefront.
A wavefront is the locus of points having the same phase of vibration. The direction of propagation of energy (the ray) is always perpendicular (normal) to the …
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