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Old Nov 15th 2008, 07:36 AM   #1
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Urgent?? line density??

Consider two monochromatic sources of light, blue light of wavelength λb = 460 nm and red light of wavelength 670 nm. The red light is shun upon a diffraction grating of density, nr = 10,000 lines per cm. A diffraction grating with what line density (lines/cm) is required to have the 2nd maximum (m=2) of blue light coincide with the 1stmaximum (m=1) of red light?


I honestly doubt if I know how to do this question.... I'm blank, someone please help? Thanks


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Old Dec 7th 2014, 11:51 AM   #2
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Answer

λb = 460 nm

λr = 670 nm

Grating formula = m x λ = d (sin of entrance angle + sin of exit angle)

I assume the entrance angle is 0°.

For red:

1 x 670 nm = 10 000 / cm x (sin 0° + sin of exit angle)

sin of exit angle = 0,67.

Meeting at the same point means having the same exit angle for both light waves. So for blue:

2 x 460 nm = ? x (sin 0° + 0,67)

? = 13 731 l / cm
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