Showing posts with label Interference & Diffraction. Show all posts
Showing posts with label Interference & Diffraction. Show all posts

What is wavelet and secondary wavefront?


Wavelets are tiny parts of waves.

Secondary wavefront:

The envelop of all the secondary wavelets in the forward direction is called the secondary wavefront. The backward envelop is not a secondary wavefront.

Define wavefront?


A wavefront can be defined as the locus of all the points of the medium which are vibrating in phase and are also displaced at the same time.

If the distance of the source is small the wavefront is spherical. But when the distance is large then any small portion of the wavefront can be considered plane.

What is diffraction grating?

Diffraction grating

Diffraction grating is an extremely useful device. In one of its form it consists of a very large number of narrow slits side by side. The slits are separated by opaque spaces.

What do you mean by Fresnel and Fraunhofer class of diffraction?

Diffraction phenomena can conveniently divided into two groups.
  1. Fresnel Diffraction Phenomena: In the Fresnel class of diffraction, the source or the screen or the both are at finite distances from the aperture or obstacle causing diffraction. In this case, the effect at a specific point on the screen due to the exposed incident wavefront is considered and no modification is made by lenses and mirrors.
  2. Fraunhofer Diffraction Phenomena: In the Frauhofer class of diffraction phenomena, the source and the screen on which the pattern is observed are at infinite distances from the aperture or the obstacle causing diffraction. The incoming light is rendered parallel with a lens and the diffracted beam is focused on the screen with another lens.

What is diffraction? Explain.


The rhythmic variations in intensity and the bending of light around the corners of an obstacle or the encroachment of light into the region of geometrical shadow constitute a class of phenomena known as the diffraction of light.

What are the Newton’s rings and how are they formed?

Newton’s rings and its formation : 

When a Plano-convex lens of long focal length is placed on a plane glass plate, a thin film of air is enclosed between the lower surface of the lens and the upper surface of the plate. The thickness of the air film is very small at the point of contact and gradually increases from the center outwards. 

What is fringe width?

Fringe width 

The distance between any two consecutive bright or dark fringe is known as fringe width. Symbolically the fringe width is β and

Why two independent source of light can not produce interference?

Two independent sources of light can not produce interference because a large number of difficulties are involved. The two sources may emit light waves of largely different amplitude and wave length and the phase difference between the two may change with time. That’s why interference of light is produced by two coherent sources which are virtual.

What do you mean by coherent sources? How are they realized in practice?

Coherent sources

Two sources are said to be coherent if they emit light waves of same frequency, nearly equal amplitude and are always in phase with each other.

Define and explain interference of light.

Interference of light: 

Wave emitted from two coherent sources having the same wavelength, equal or nearly equal amplitude when passes through a point in a medium, then superposition takes place. 

During superposition, if the two waves reaching the point in the same phase then the intensity will be maximum that is bright. But if the two waves reaching the point in opposite phase then the point will have minimum intensity that is dark. This variation of intensity due to superposition is called interference of light.

The interference is said to be constructive or destructive when the intensity is respectively more or less than that given by one beam alone.

Explain Huygens principles.

Huygens principle

To explain the propagation of light through ether, Huygens proposed the following principles for the construction of a wavefront at a subsequent time from the known position of the wavefront at any given instance.