Q.Read the following passage carefully and answer the questions below: "In 1678, the Dutch physicist Christiaan Huygens proposed the wave theory of light. His wave theory is founded on the concept of the wavefront. Light travels in the form of waves. The wavefront is the locus traced by the source points of secondary wavelets vibrating in the same phase. Hence a wavefront may be defined as a surface of constant phase. The speed with which the wavefront spreads outward from the source is called the speed of the wave."

Showing the 12 most recent of 26 on this concept.

1. CBSE 2026Set V11 markMCQ
   Q.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 …

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2. CBSE 2026Set A1 markMCQ
   Q.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 ** …

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3. CBSE 2026Set ANNUAL1 markMCQ
   Q.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 …

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4. CBSE 2026Set ANNUAL1 markMCQ
   Q.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 …

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5. CBSE 2026Set ANNUAL1 markMCQ
   Q.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 …

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6. CBSE 2026Set ANNUAL1 mark
   Q.Draw the diagram of wave front produced by a point source of light.

   ›Reveal solutionSolution

   

   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 …

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7. CBSE 2025Set ANNUAL1 markMCQ
   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.

   (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 …

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8. CBSE 2025Set ANNUAL1 mark
   Q.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. …

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9. CBSE 2025Set ANNUAL1 markMCQ
   Q.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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10. CBSE 2025Set ANNUAL1 markMCQ
    Q.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 …

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11. CBSE 2024Set A11 markMCQ
    Q.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 …

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12. CBSE 2024Set ANNUAL1 markMCQ
    Q.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$^\circ$ 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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