4-9. Two narrow slits separated by 0.30 mm are illuminated with light of wavelen

Question

4-9. Two narrow slits separated by 0.30 mm are illuminated with light of wavelength 496 nm. (a) How far are the first three bright fringes from the center of the pattern if observed on a screen 120 cm distant? (b) How far are the first three dark fringes from the center of the pattern ?

4-18. A diffraction grating is made up of slits of width 300 nm with a 900 nm separation between their centers. The grating is illuminated by monochromatic plane waves, ? = 600 nm, the angle of incidence being zero. (a) How many diffraction maxima are there? (b) What is the angular width of the spectral lines observed, if the grating has 1000 slits? Angular width is defined to be the angle between the two positions of zero intensity on either side of the maximum. ?

A beam of laser light with an energy flux S of 10 W/cm2 falls for 1.0 h on a perfectly reflecting plane mirror of 1.0-cm2 area. (a) What momentum is delivered to the mirror in this time and (b) what force acts on the mirror?

Explanation / Answer

4.19

As

fringe spacing = wavelength * screen distance / slit separation

Thus,

fringe spacing = 1.984 mm = delta y

Thus, the first three bright fringes are

delta y, 2delta y, 3delta y = 1.984 mm, 3.968 mm, 5.952 mm from the central bright fringe.   [ANSWER]

B.

Also, the first three dark fringes are

0.5delta y, 1.5delta y, 2.5delta y = 0.992 mm, 2.976mm, 4.96 mm   [ANSWER]

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