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In Fraunhofer diffraction experiment, L is the distance between screen and the obstacle, b is the size of obstacle and λ . is wavelength of incident light. The general condition for the applicability of Fraunhofer diffraction is

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Important Questions on Optics

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Physics>Optics>Optics>Diffraction
In a diffraction pattern due to a single slit of width a the first minimum is observed at an angle 30o when light of wavelength 5000 Å is incident on the slit. The first secondary maximum is observed at an angle of:
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The box of a pin hole camera, of length L, has a hole of radius a. It is assumed that when the hole is illuminated by a parallel beam of light of wavelength λ the spread of the spot (obtained on the opposite wall of the camera) is the sum of its geometrical spread and the spread due to diffraction. The spot would then have its minimum size (say bmin ) when:
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Physics>Optics>Optics>Diffraction
The luminous border that surrounds the profile of a mountain just before sun rises behind it, is an example of
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Physics>Optics>Optics>Diffraction
A parallel beam of electrons travelling in x - direction falls on a slit of width d (see figure). If after passing the slit, an electron acquires momentum py in the y - direction then for a majority of electrons passing through the slit (h is Planck's constant):

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A single slit of width b is illuminated by a coherent monochromatic light of wavelength λ. If the second and fourth minima in the diffraction pattern at a distance 1 cm from the slit are at 3 cm and 6 cm respectively from the central maximum, what is the width of the central maximum? (i.e. distance between first minimum on either side of the central maximum)
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In the figure below, P and Q are two equally intense coherent sources emitting radiation of wavelength 20m. The separation between P and Q is 5m and the phase of P is ahead of that of Q by 90°.A,B and C are three distinct point of observation, each equidistant from the midpoint of PQ. The intensities of radiation at A,B,C will be in the ratio :

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Physics>Optics>Optics>Diffraction
A beam of light of λ=600 nm from a distant source falls on a single slit 1 mm wide and the resulting diffraction pattern is observed on a screen 2 m away. The distance between first dark fringes on either side of the central bright fringe is:
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The penetration of light into the region of geometrical shadow is called
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Physics>Optics>Optics>Diffraction
At the first minimum adjacent to the central maximum of a single-slit diffraction pattern, the phase difference between the Huygen's wavelet from the edge of the slit and the wavelet from the midpoint of the slit is:
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Physics>Optics>Optics>Diffraction
A single slit of width 0.1 mm is illuminated by a parallel beam of light of wavelength 6000 Å and diffraction bands are observed on a screen 0.5 m from the slit. The distance of the third dark band from the central bright band is:
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Physics>Optics>Optics>Diffraction
The wave nature of electrons was experimentally verified by,
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Physics>Optics>Optics>Diffraction
A linear aperture whose width is 0.02 cm is placed immediately in front of a lens of focal length 60 cm. The aperture is illuminated normally by a parallel beam of wavelength 5×10-5 cm. The distance of the first dark band of the diffraction pattern from the centre of the screen is
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Physics>Optics>Optics>Diffraction
A Fraunhofer diffraction pattern due to a single slit of width 0.3 mm is obtained on a screen placed at a distance of 3 m from the slit. The first minima lie at 5.5 mm on either side of the central maximum on the screen. The wavelength of light used is
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Physics>Optics>Optics>Diffraction
Visible light of wavelength 6000×10-8cm falls normally on a single slit and produces a diffraction pattern. It is found that the second diffraction minimum is at 60o from the central maximum. If the first minimum is produced at θ1, then θ1 is close to
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Physics>Optics>Optics>Diffraction
The bending of beam of light around corners of obstacles is called
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Physics>Optics>Optics>Diffraction
Consider the diffraction pattern obtained from the sunlight incident on a pinhole of diameter 0.1 μm. If the diameter of the pinhole is slightly increased, it will affect the diffraction pattern such that
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Physics>Optics>Optics>Diffraction
A screen is placed 2 m away from a narrow slit. If the first minimum lies 5 mm from either side of the central maxima, when plane waves of wave length 5×10-7 m are used, the slit width is:
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Physics>Optics>Optics>Diffraction
The angular width of the central maximum in a single slit diffraction pattern is 60° . The width of the slit is 1 μm. The slit is illuminated by monochromatic plane waves. If another slit of the same width is made near it, Young's fringes can be observed on a screen placed at a distance 50 cm from the slits. If the observed fringe width is 1 cm, what is slit separation distance? (i.e., the distance between the centres of each slit.)
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Physics>Optics>Optics>Diffraction
A parallel beam of light of wavelength 4000 A∘ passes through a slit of width 5×10-3 m. The angular spread of the central maxima in the diffraction pattern is
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Physics>Optics>Optics>Diffraction
For a parallel beam of monochromatic light of wavelength λ, diffraction is produced by a single slit whose width a is of the order of the wavelength of the light. If D is the distance of the screen from the slit, the width of the central maxima will be: