Contents
With new discoveries and innovations constantly being made, the study of Physics Topics remains a vibrant and exciting field of research.
What are the Types of Wavefront? What do you Mean by a Ray?
Introduction
Light propagates through vacuum or any medium in the form of waves. Light wave is a transverse electromagnetic wave. Optical phenomena observed in daily life, as produced by slits or orifices, obstacles, reflecting planes, refracting planes, have sizes many times more than the wavelength of light. For example, wavelength of visible light ranges from 4 × 10-5 cm to 8 × 10-5 cm, but even a very small slit in the path of light generally has a diameter not less than 1 mm i.e., 0.1 cm.
Hence, diameter of the orifice (slit) is 1000 times more than the wavelength of visible light. Wavelength of light is taken as zero, with respect to large orifices, slits, obstacles, reflecting surfaces or refracting surfaces, i.e., light is not considered as waves. However in cases, when the size of an orifice or obstacle, in the path of light is too small and is comparable to the wavelength of light, the wavelength of visible light can no more be taken as zero
Optics: This branch of physics deals with properties of light. Optics is divided in two parts
- geometrical optics and
- physical optics.
Geometrical optics: In this branch of optics, it is assumed that the wavelength of light is negligible in comparison with the sizes of instruments used in experiments (like an orifice or an obstacle).
Physical optics: It is assumed that sizes of instruments used in experiment (like size of an orifice or an obstacle) are comparable with the wavelength of light.
Wave Front
Concept of wavefront: When a stone is dropped in a reservoir, waves are set up on its still water surface. From the centre of disturbance, i.e., where the stone is dropped, waves spread out in all directions at constant velocity. Water particles however, do not spread out horizontally with the wave. Instead, they vibrate vertically. Let us assume that at an instant of time the displacement of a water particle, at some distance from the centre of disturbance, is maximum. At that moment, displacements of all water particles, situated on the circumference of the circle of radius same as the distance of the point in reference, are maximum.
This implies that water particles on the circumference of all circles, concentric with the circle described above, should be in the same phase. So, wave spreads on the surface of a medium in the form of circles. Hence, while spreading of a wave in a three-dimensional medium, if a sphere is imagined with the source of disturbance as its centre, all points lying on the surface of this imaginary sphere, are in the same phase. In this case a spherical wavefront is obtained, if the radius of spherical wavefront is large enough, a small part of the wavefront can be taken as plane wavefront.
Definition: As a wave generated from a source spreads in all directions through vacuum or a medium, the locus (line or surface) of the points in the path of wave which are in equal phase at any moment is called a wavefront.
Different types of wavefronts:
i) Spherical wavefront: The wavefront formed during the propagation of light coming from a point source is considered spherical In shape. The locus of the points in same phase is spherical. This is known as spherical wavefront
ii) Cylindrical wavefront: The wavefront produced during the propagation of light coming from a line source (e.g. single slit) is considered cylindrical in shape [Fig.].
iii) Plane wavefront: The rays coming from infinity are parallel and the wavefront thus associated can be considered plane wavefront [Fig.].
Properties of wavefronts:
- A perpendicular drawn at any point on a wavefront, shows the direction of velocity of the wave at that point.
- Velocity of a wave actually denotes velocity of the wave-front. If v is the wave velocity, velocity of each wavefront is also v.
- Whether it is a spherical or a plane wavefront, the perpendicular distance between two consecutive wavefronts in the same phase is called wavelength of that wave.
The phase difference between any two points located on the same wavefront is zero.
Ray: Perpendicular drawn to the wavefront is called a ray. Energy of a wave is transmitted from one part to another in vacuum or in a medium along these rays. Direction of rays are shown in Fig.’s by yellow arrow head.