Physics · Wave Optics · NEET
A secondary wavelet is a tiny spherical wave sent out by a single point on the existing wavefront. Huygens said every point on a wavefront behaves like a fresh point source. If the wave speed in the medium is v, then after a time t each of these point sources has sent out a sphere of radius v times t. All these little spheres are the secondary wavelets.
You draw all the secondary wavelets (spheres of radius v times t from each point), then draw the surface that just touches all of them on the forward side. This common tangent surface is called the envelope, and it is the new wavefront after time t. For a plane wavefront you get a new plane; for a spherical wavefront you get a new sphere farther out.
Each secondary wavelet is a full sphere, so in theory it also has a backward part. But experiments show no wave travels backward. Huygens assumed only the forward envelope is real. Later Fresnel and Kirchhoff explained this with an obliquity factor that makes the amplitude of the backward wavelet effectively zero. For NEET, just remember: take only the FORWARD tangent.
No. It is a general geometric rule for any wave: light, sound, water waves, or any electromagnetic wave. Huygens first used it to explain reflection and refraction of light, but the same secondary-wavelet construction works for all waves that travel through a medium (or vacuum, for light).
Not fully. Huygens principle is a geometric construction that predicts how a wavefront moves and gives the laws of reflection and refraction. The idea of interference (constructive and destructive addition of amplitudes) was added later by Young and Fresnel. So Huygens explains propagation and bending, but not the bright and dark fringes on its own.
The primary wavefront is the original wavefront you start with at time zero. The secondary wavelets are the tiny spheres each point of that primary wavefront emits. The new wavefront (the forward tangent of all wavelets) after time t is sometimes called the secondary wavefront. So: primary front to wavelets to new (secondary) front.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Christiaan Huygens, a Dutch physicist, proposed it in his book Treatise on Light. He is called the founder of the wave theory of light.
Every point on a wavefront is a source of secondary wavelets that spread out with the wave speed, and the forward tangent (envelope) to these wavelets gives the new wavefront at a later time.
It equals the speed of the wave in that medium, v. After time t, the radius of each secondary wavelet is v times t.
When the wave speed is the same in all directions (isotropic medium), the secondary wavelets are spherical. The rays are then perpendicular to the wavefront.
It is the base idea used to derive the laws of reflection, refraction (Snell's law), and total internal reflection using wave theory. Understanding it makes the whole Wave Optics chapter easier.
