15.1 Wavefront
To understand how energy is spread out in the form of waves, we need to understand the concept of wavefront. A wavefront is an imaginary surface that connects all the points where the waves have the same phase.
Wavefronts can be of three types:
- Plane wave fronts: Represented as a straight line like light waves from the distant stars and water waves in a ripple tank.
- Circular wavefronts: Represented by circles as formed in water inside a pond.
- Spherical wavefronts: Formed in three-dimensional space by a sound or light wave.
For a point source in space, the wavefronts will always be spherical i.e. they spread out in all directions. For very large distances, like light rays coming from sun, these wavefronts can be assumed as planes on the surface of earth. The shortest distance between wavefronts is always equal to wavelength of wave. Direction of transfer of energy of wave is always perpendicular to any point on the surface of a given wavefront.
(Fig. 15.1(a,b,c): Plane and circular waves Parts of spherical wavefronts. For very large distance from source, each part becomes a plane)
If the source is periodic, it produces a succession of wavefronts, all of the same shape. Plane wavefronts are produced by a plane source or by any source at distant point. A line source for example a vibrator in a ripple tank creates a wavefronts that are plane and straight in two dimensions. A point source S generates circular warfronts in two dimensions and spherical wavefronts in three dimensions as shown in Fig. 15.1, A line at right angle to a wavefront which shows its direction of travel is called a ray, as shown in Fig. 15.1 (a).
Here in this chapter, we will assume plane wavefronts from different sources to study interference and diffraction phenomenon. If a source of light is placed at the focal point of a lens. Plane wavefronts are formed on the other side of lens.