phy- diffraction

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Larry

2011-02-19 5:30 pm

When a source of monochromatic light of wavelength 6x10^-7 m is viewed 2 m away through a diffraction grating, two similar lamps are ‘seen’ one on each side of it. Each is 0.3 m away from it. Calculate the number of lines in the grating.

更新1:

why for 1st order fringe?

更新2:

but for small angle (theta), sin(theta) = tan(theta) = 0.3/2

更新3:

why[but for small angle (theta), sin(theta) = tan(theta) = 0.3/2]?

回答 (2)

天同

2011-02-19 6:30 pm

✔ 最佳答案

Using the grating equation for bright fringes
a.sin(theta) = m入
where a is the grating line spacing
(theta) is the angular position of a bright fringe of order m
入 is the wavelength of light used.

hence, sin(theta) = 6 x 10^-7/a for 1st order fringe
but for small angle (theta), sin(theta) = tan(theta) = 0.3/2
i.e. 0.3/2 = 6x 10^-7/a
a = 4x10^-6 m

Hence, no. of lines in the grating = 1/a = 1/4x10^-6 lines/m =2.5x10^5 lines/m

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2011-02-19 7:32 pm

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