8.4.1—D sin θ = nλ

For a grating with slit spacing d, constructive maxima occur when d sinθ=nλ, where n is an integer order.

Use the grating line density to find d, measure θ from the central maximum and check that |sinθ|≤1.

A 600 lines mm⁻¹ grating has d=1.67 μm; first order at θ=22° gives a wavelength near 6.2×10⁻⁷ m.

n labels bright orders, not the number of slits; the central maximum is n=0.