(a) Define a wavefront. Using Huygens' geometrical construction, explain with the help of a diagram how the plane wavefront travels from the instant $t_1$ to $t_2$ in air. (b) A plane wavefront is incident on a convex lens. Explain, with the help of the diagram, the shape of the refracted wavefront formed.

(a) Define a wavefront. Using Huygens' geometrical construction, explain with the help of a diagram how the pl

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(b) A plane wavefront is incident on a convex lens. Explain, with the help of the diagram, the shape of the refracted wavefront formed.

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Diagram showing a plane wavefront refracted by a thin prism into a converging spherical wave, and a plane wavefront refracted by a thin convex lens into a converging spherical wavefront that focuses at point F.

Huygens' geometrical construction showing a plane wavefront F1F2 at time t=0 propagating to a new plane wavefront G1G2 at time t=tau, built from the envelope of secondary spherical wavelets emitted from points A1, B1, C1, D1 on the original wavefront.

A wavefront is a surface of constant phase; Huygens' principle treats every point on it as a secondary source, and the envelope of these wavelets at a later time gives the new wavefront. For a plane wave, the envelope is another parallel plane. A convex lens delays the central part more than the edges, converting the plane wavefront into a converging spherical wavefront.

(a) Wavefront definition and Huygens' construction for a plane wave

1. What is a wavefront?

A wavefront is the continuous locus of all points in a medium that are vibrating in the same phase. In a plane wave, these points lie on parallel planes perpendicular to the direction of propagation. The distance between successive wavefronts equals the wavelength $\lambda$ .

2. Huygens' principle — the core idea

Christiaan Huygens proposed that every point on a given wavefront acts as a source of secondary spherical wavelets (disturbances). These wavelets travel outward with the wave speed $v$ in the medium. After a time $\Delta t = t_2 - t_1$ , the new wavefront is the tangent envelope (common tangent surface) to all these secondary wavelets.

Tip

Think of a row of stones dropped into a pond at the same instant — each creates an expanding circular ripple. The line that just touches all these circles at a given moment is the new wavefront.

3. Step-by-step construction for a plane wavefront

Initial wavefront at $t_1$ : Consider a plane wavefront $AB$ (a straight line in 2D) moving perpendicular to its plane in air. All points on $AB$ have the same phase.
Secondary sources: Choose several points $P_1, P_2, P_3, \dots$ on $AB$ . Each point emits a spherical wavelet of radius $r = v \cdot (t_2 - t_1)$ .
Draw the wavelets: With each chosen point as centre, draw arcs of radius $r$ on the side toward which the wave is travelling.
Find the envelope: Draw a line $A'B'$ that is tangent to all these arcs. Since the wavelets have equal radii and the original wavefront is straight, the tangent line is also straight and parallel to $AB$ .
Result: The new wavefront at $t_2$ is the plane $A'B'$ , shifted forward by distance $r = v\Delta t$ .

Watch out

A common mistake is to think the wavelets themselves are the new wavefront. They are not — the new wavefront is the common tangent to all wavelets, not the sum of their amplitudes.

Diagram description (for your sketch):

Draw a horizontal line $AB$ (wavefront at $t_1$ ). Below it, mark points $P_1, P_2, P_3$ along $AB$ . From each, draw a semicircle of equal radius downward. Draw a line $A'B'$ that just touches all semicircles at their lowest points. Label the distance between $AB$ and $A'B'$ as $v(t_2 - t_1)$ . Arrows perpendicular to $AB$ show the direction of propagation.

(b) Plane wavefront incident on a convex lens

1. What happens at the lens?

A convex lens is thicker at the centre than at the edges. Light travels slower in glass than in air (refractive index $n > 1$ ). The central part of the wavefront passes through more glass, so it is delayed more than the edges. This differential delay bends the wavefront.

2. Applying Huygens' principle through the lens …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 26 on this concept.

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

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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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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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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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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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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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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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Q.(a) Define a wavefront. Using Huygens' geometrical construction, explain with the help of a diagram how the plane wavefront travels from the instant $t_1$ to $t_2$ in air.

Concept understanding — Huygens' Principle and Wavefronts

(a) Wavefront definition and Huygens' construction for a plane wave

(b) Plane wavefront incident on a convex lens

Q.(a) Define a wavefront. Using Huygens' geometrical construction, explain with the help of a diagram how the plane wavefront travels from the instant t1t_1t1​ to t2t_2t2​ in air.

(a) Wavefront definition and Huygens' construction for a plane wave

(b) Plane wavefront incident on a convex lens

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Q.(a) Define a wavefront. Using Huygens' geometrical construction, explain with the help of a diagram how the plane wavefront travels from the instant $t_1$ to $t_2$ in air.

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