Diffraction grating orders and spectral dispersion
At normal incidence on a periodic grating of pitch d, principal propagating orders satisfy d sinθ_m=mλ.
Key knowledge
At normal incidence on a periodic grating of pitch d, principal propagating orders satisfy d sinθ_m=mλ. Their angular wavelength dispersion is dθ_m/dλ=m/(d cosθ_m); finite illuminated extent gives finite peak width, and the groove/aperture profile controls relative order strengths.
Values and equations
d sinθ_m=mλ; dθ_m/dλ=m/(d cosθ_m)
- d,λ
- Grating pitch and wavelength m
- θ_m
- Order angle rad
- m
- Signed integer order
- dθ_m/dλ
- Angular wavelength dispersion rad m⁻¹
Conditions and scope
- Uniform periodic grating, wavelength in the surrounding medium, normal incidence and |mλ/d|≤1; derivative away from grazing cosθ=0.
- Oblique incidence has an incident-angle term. The order condition alone does not determine diffraction efficiency or a universal spectral resolving power.
Sources · 2
References
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Evidence and review
University Physics 3: 4-4-diffraction-gratings: Normal-incidence grating equation in applications/example; finite slit count and narrow maxima. Optics 2.71: Diffraction lecture 14 (2014): Pages 1–2: quadratic propagation phase and Fraunhofer limit; pages 4–6: lens Fourier plane and normal-incidence grating equation. Scalar source normalization is supplied by the separate recitation kernel.
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Entry details
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