Wave Optics | Class 12 Physics Notes
By ConceptScroll Team · Published on 17 July 2026 · 3 min read
Wave Optics – this guide gives you a concise, exam-ready overview of Wave Optics from Class 12 Physics, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
10.2 Huygens PRINCIPLE
Huygens' principle is a fundamental concept in wave optics that helps determine the position and shape of wavefronts at any time. A wavefront is defined as a surface of constant phase where all points oscillate in phase. For example, when a stone is dropped in water, circular wavefronts spread out from the point of impact, with all points on a circle oscillating in phase.
For a point source emitting waves uniformly in all directions, the wavefronts are spherical surfaces centered on the source, called spherical waves. At large distances, a small portion of a spherical wavefront can be approximated as a plane wave.
Huygens' principle states that every point on a wavefront acts as a source of secondary wavelets that spread out in all directions with the speed of the wave. The new wavefront at a later time is the envelope (common tangent) of all these secondary wavelets.
For example, consider a spherical wavefront at time t=0. Drawing spheres of radius vτ (v is wave speed, τ is time interval) from each point on the wavefront, the common tangent to these spheres gives the new wavefront at time t=τ. This construction can be used for spherical and plane waves.
One subtlety is the absence of a backward wave (backwave) in the wavefront propagation. Huygens assumed the amplitude of secondary wavelets is maximum in the forward direction and zero backward, an ad hoc assumption later justified by rigorous wave theory.
The rays of light are defined as lines normal to the wavefronts, indicating the direction of energy propagation. Huygens' principle provides a geometric method to understand wave propagation and forms the basis for deriving laws of reflection and refraction.
📊 Diagram: Figure 10.2 F1F2 represents the spherical wavefront (with O as centre) at t = 0. The envelope of the secondary wavelets emanating from F1F2 produces the forward moving wavefront G1G2. The backwave D1D2 does not exist; Figure 10.3 Huygens geometrical construction for a plane wave propagating to the right. F1F2 is the plane wavefront at t = 0 and G1G2 is the wavefront at a later time τ. The lines A1A2, B1B2, etc., are normal to both F1F2 and G1G2 and represent rays.
🔗 Connection: Prepares for Section 10.3 where Huygens' principle is applied to derive laws of reflection and refraction of plane waves.
Frequently asked questions
Laser light is considered to be coherent because it consists of
coordinated waves of exactly the same wavelength
Four lenses of focal length + 15 cm and + 150 cm are available for making a telescope. To produce the maximum magnification, the focal length of the eyepiece should be
+15 cm
Diffraction pattern is obtained using red light. What happens when the red light is replaced by the blue light?
the diffraction band becomes narrow and crowded
. In young's double slit experiment, the fringe pattern as seen on the screen is
hyperbola
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