What is most likely the color of the light whose second-order bright band forms an angle of 13.5° if the diffraction grating has 175 lines per mm?

For a diffraction grating, bright bands satisfy d sin θ = mλ. The grating spacing is 1/175 mm, or about 5714 nm, and the second order means m = 2. Thus λ = (5714 nm × sin 13.5°)/2 ≈ 667 nm, which is in the red part of the visible spectrum.

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