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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.
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संबंधित प्रश्न
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.
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What must be the ratio of the slit width to the wavelength for a single slit, to have the first diffraction minimum at 45˚?
In biprism experiment, the distance between source and eyepiece is 1.2 m, the distance between two virtual sources is 0.84 mm. Then the wavelength of light used if eyepiece is to be moved transversely through a distance of 2.799 cm to shift 30 fringes is ______.
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 ______.
A beam of light of wavelength 600 nm from a distant source falls on a single slit 1 mm wide and the resulting diffraction pattern is observed on a screen 4 m away. The distance between the first dark fringes on either side of the central bright fringe is ______.
For Fraunhofer diffraction to occur ____________.
In a diffraction pattern, width of a fringe ____________.
In a single slit diffraction pattern, which of the following is incorrect for fringe pattern?
The condition for observing Fraunhofer diffraction pattern from an obstacle is that the light wavefront incident on it must be ______.
The diffraction fringes obtained by a single slit are of ____________.
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(Y = Young's modulus of the material of metal bar)
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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 using monochromatic light of wavelength λ, the maximum intensity of light at a point on the screen is K units. The intensity of light at point where the path difference is `lambda/3` is ______.
`[cos60^circ = sin30^circ 1/2]`
The first diffraction minimum due to single slit diffraction is θ, for a light wavelength 5000 Å. If the width of slit is 1 × 10-4 cm, then the value of θ is ______.
In a double slit interference experiment, the distance between the slits is 0.05 cm and screen is 2 m away from the slits. The wavelength of light is 6000 Å. The distance between the fringes is ______.
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 ______.
Using the geometry of the double slit experiment, derive the expression for fringe width of interference bands.
