- Fraunhofer diffraction produced by light falling normally from a slit.
Condition of intensity minima:
b sin 0 = k = 1, 2, 3,.. (5.3b)
where b is the width of the slit, 0 is the diffraction angle. - Diffraction grating, with light falling normally. The main Fraunhofer
maxima appear under the condition
d sin 0 = ±k?, k = 0, 1, 2,^ (5.3c)
the condition of additional minima:
d sin 0= ±2n. ,
where k' = 1, 2,.. ., except for 0, N, 2N ,....- Angular dispersion of a diffraction grating:
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D = (^) d cos° '
- Resolving power of a diffraction grating:
R = T.2:=kN ,(5.3d)(5.3e)(5.3f)where N is the number of lines of the grating.- Resolving power of an objective
1
R — 6 11) 1. 22 ' (5.3g)
where 611) is the least angular separation resolved by the objective, D is the
diameter of the objective.- Bragg's equation. The condition of diffraction maxima:
2d sin a = +/A, (5.3h)
where d is the interplanar distance, a is the glancing angle, k = 1, 2, 3,....
5.97. A plane light wave falls normally on a diaphragm with
round aperture opening the first N Fresnel zones for a point P on
a screen located at a distance b from the diaphragm. The wave-
length of light is equal to 2n ,. Find the intensity of light /^0 in front
of the diaphragm if the distribution of intensity of light I (r) on the
screen is known. Here r is the distance from the point P.
5.98. A point source of light with wavelength X = 0.50 p,m is
located at a distance a = 100 cm in front of a diaphragm with
round aperture of radius r = 1.0 mm. Find the distance b between
the diaphragm and the observation point for which the number of
Fresnel zones in the aperture equals k = 3.
5.99. A diaphragm with round aperture, whose radius r can be
varied during the experiment, is placed between a point source of
light and a screen. The distances from the diaphragm to the source
and the screen are equal to a = 100 cm and b = 125 cm. Determine
the wavelength of light if the intensity maximum at the centre of
the diffraction pattern of the screen is observed at r, = 1.00 mm
and the next maximum at r 2 = 1.29 mm.
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