Physics · Wave Optics · NEET
No. A wave is the actual travelling disturbance (the energy moving through the medium). A wavefront is just an imaginary surface we draw to mark all the points of that wave which are in the same phase at one instant. One wave has many wavefronts - for example, all the crests form one set of wavefronts and all the troughs form another set. The wavefront is a snapshot tool that helps us see the shape and direction of the wave.
Light from a point source spreads out equally in all directions, so points at the same distance form the surface of a sphere - that is a spherical wavefront. As the wavefront travels far from the source, the sphere becomes huge. A small patch of a very large sphere looks almost flat, just like a small area of the Earth looks flat to us. So at large distances the wavefront is treated as a plane wavefront. That is why sunlight, coming from about 150 million km away, reaches us as plane wavefronts.
Both are linked for a wave in one uniform medium. Points on the same wavefront are in the same phase, and in a single medium they are also at the same distance from the source (because the wave travels at the same speed in every direction). But the fundamental defining property is SAME PHASE. If the medium is not uniform, the equal-distance rule can break, but the equal-phase rule is what always defines a wavefront.
You get a cylindrical wavefront when the source is a straight LINE instead of a single point. For example, light coming out of a thin long slit spreads out around that line, so points in the same phase lie on the curved surface of a cylinder. A point source gives spheres; a line source gives cylinders; a very distant or plane source gives flat planes.
A ray is a line that shows the direction in which the wave energy travels. A wavefront is a surface of equal phase. The important rule for NEET: the ray is always PERPENDICULAR to the wavefront. So for a spherical wavefront the rays point outward like radius lines (radial), and for a plane wavefront the rays are parallel straight lines pointing forward. This link is covered fully in the next concept, ray vs wavefront.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is an imaginary surface that connects all the points of a wave that are moving up and down together, that is, in the same phase, at a given moment.
Spherical (from a point source), cylindrical (from a line or slit source), and plane (from a very distant source or a plane source).
Plane. The Sun sends out spherical wavefronts, but after travelling about 150 million km, the small part reaching Earth is essentially flat, so we treat it as a plane wavefront.
Perpendicular. The ray, which shows the direction of energy flow, always crosses the wavefront at 90 degrees.
Only when the wave moves through a single uniform medium at the same speed in all directions. The always-true property is that they share the same phase.
