Advertisements
Advertisements
प्रश्न
What is the diffraction of light? How does it differ from interference? What are Fraunhofer and Fresnel diffractions?
उत्तर
The phenomenon of diffraction of light:
The principle of rectilinear propagation of light from geometrical optics predicts a sharp shadow when light passes by the edge of an obstacle or through a small opening or a narrow slit and falls on a screen. However, it is discovered that some of the light deviates from its rectilinear path and enters the geometrical shadow region. This is a common feature of wave phenomena that occurs when a portion of the wavefront is obstructed in some way. This bending of light waves at an edge into the region of the geometrical shadow is called the diffraction of light.
Differences between interference and diffraction:
- Interference is the term used to describe the superposition of a few coherent waves (say, two). But when a large number of waves from different parts of the same wavefront superimpose at a point, the effect is known as diffraction.
- All double-slit interference fringes are the same width. Only the non-central maxima in a single-slit diffraction pattern are of equal width, which is half the width of the central maximum.
- The bright and dark fringes are equally spaced in double-slit interference. Only the non-central maxima in diffraction lie approximately half way between the minima.
- In double-slit interference, bright fringes are of equal intensity. In diffraction, successive noncentral maxima decrease rapidly in intensity.
Diffraction can be classified into two types depending on the distances involved in the experimental setup:
- Fraunhofer diffraction:
If the distances between the primary source of light, the obstacle/slit causing diffraction and the screen for viewing the diffraction pattern are very large, the diffraction is called Fraunhofer diffraction. In this case, the wavefront incident on the obstacle can be considered to be a plane wavefront. For this, we generally place the source of light at the focus of a convex lens so that a plane wavefront is incident on the obstacle and another convex lens is used on the other side of the obstacle to make the pattern visible on the screen.
Set up for Fraunhofer diffraction - Fresnel diffraction:
In this case, the distances are much smaller and the incident wavefront is either cylindrical or spherical depending on the source. A lens is not required to observe the diffraction pattern on the screen.
APPEARS IN
संबंधित प्रश्न
In Fraunhoffer diffraction by a narrow slit, a screen is placed at a distance of 2 m from the lens to obtain the diffraction pattern. If the slit width is 0.2 mm and the first minimum is 5 mm on either side of the central maximum, find the wavelength of light.
In a biprism experiment, the fringes are observed in the focal plane of the eyepiece at a distance of 1.2 m from the slits. The distance between the central bright and the 20th bright band is 0.4 cm. When a convex lens is placed between the biprism and the eyepiece, 90 cm from the eyepiece, the distance between the two virtual magnified images is found to be 0.9 cm. Determine the wavelength of light used.
What must be the ratio of the slit width to the wavelength of light for a single slit to have the first diffraction minimum at 45.0°?
A star is emitting light at the wavelength of 5000 Å. Determine the limit of resolution of a telescope having an objective of a diameter of 200 inch.
In a single slit diffraction pattern, the first minima obtained with the red light of wavelength 6600 A.U. coincides with the first maxima f some other wavelength λ then is ______
What must be the ratio of the slit width to the wavelength for a single slit, to have the first diffraction minimum at 45˚?
What is the difference between Fresnel and Fraunhofer diffraction?
Explain experimental setup for Fraunhofer diffraction with neat diagram.
Explain Fraunhofer diffraction at a single slit with a neat ray diagram. Obtain an expression for the width of the central bright fringe.
A lens having focal length f gives Fraunhofer type diffraction pattern of a slit having width a. If wavelength of light is λ, the distance of first dark band and next bright band from axis is given by ____________.
Light of wavelength 'λ' is incident on a single slit of width 'a' and the distance between slit and screen is 'D'. In diffraction pattern, if slit width is equal to the width of the central maximum then 'D' is equal to ______.
A slit of width a is illuminated by white light. For red light `(λ = 6500 Å)`, the first minima is obtained at θ = 60°. Then the value of a will be ______.
The luminous border that surrounds the profile of a mountain just before sun rises behind it, is an example of ______.
In a single slit diffraction pattern, which of the following is incorrect for fringe pattern?
The diffraction fringes obtained by a single slit are of ____________.
A parallel beam of monochromatic light falls normally on a single narrow slit. The angular width of the central maximum in the resulting diffraction pattern ______
A parallel beam of monochromatic light of wavelength 5 × 10-7 m is incident normally on a single narrow slit of width 10-3 mm. At what angle of diffraction, the first minima are observed?
In young 's double slit experiment the two coherent sources have different amplitudes. If the ratio of maximum intensity to minimum intensity is 16 : 1, then the ratio of amplitudes of the two source will be _______.
The increase in energy of a metal bar of length 'L' and cross-sectional area 'A' when compressed with a load 'M' along its length is ______.
(Y = Young's modulus of the material of metal bar)
In Fraunhofer diffraction pattern, slit width is 0.2 mm and screen is at 2 m away from the lens. If wavelength of light used is 5000 Å, then the distance between the first minimum on either side of the central maximum is ______. (θ is small and measured in radian)
Let a steel bar of length l, breadth b and depth d be loaded at the centre by a load W. Then the sag of bending of beam is ______.
(Y = Young's modulus of material of steel)
In Fraunhoffer diffraction experiment, L is the distance between the screen and the obstacle, b is the size of the obstacle and λ is the wavelength of the incident light. The general condition for the applicability of Fraunhoffer diffraction is ______.
In a Young's double slit experiment carried out with light of wavelength λ = 5000 Å, the distance between the slits is 0.2 mm and the screen is at 200 cm from the slits. The central maximum is at x = 0. The third maximum (taking the central maximum as zeroth maximum) will be at x equal to ______.
In Young's double slit experiment, which of the following graph represents the correct variation of fringe width β versus distance D between sources and screen?
Two coherent monochromatic light beams of amplitudes E10 and E20 produce an interference pattern. The ratio of the intensities of the maxim a and minima in the interference pattern is ______.
In a biprism experiment, the slit is illuminated by red light of wavelength 6400 A and the crosswire of eyepiece is adjusted to the centre of 3rd bright band. By using blue light it is found that 4th bright band is at the centre of the cross wire. Calculate the wavelength of blue light.
Using the geometry of the double slit experiment, derive the expression for fringe width of interference bands.
