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Homework Statement
A diffraction grating having 500 lines/mm diffracts visible light at 30°. What is the light's wavelength?
d = 1/500(lines/mm) = 2x10-6(m)
θ = 30° = pi/6 rad⇔
Homework Equations
d*sin(θm) = mλ
The Attempt at a Solution
when i rearranged the equation, i got λ = d*sin(θm)/m
And the problem was how can i find what is the order of the diffraction?
I assumed it to be the first order, so m=1, and i got the wavelength was 1000nm but the answer was 500nm, so it must be the second order, m=2. But i have no idea how to get m=2