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Diffraction Grating Anomalies

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Diffraction Grating Anomalies

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  • Author: R. W. Wood
In 1902. R. W. Wood observed that the intensity of light diffracted by a grating generally changed slowly as the wavelength was varied, but occasionally a sharp change in intensity was observed at certain wavelengths.104 Called anomalies, these abrupt changes in the grating efficiency curve were later categorized into two groups: Rayleigh anomalies and resonance anomalies.Lord Rayleigh predicted the spectral locations where a certain set of anomalies would be found: he suggested that these anomalies occur when light of a given wavelength λ' and spectral order m' is diffracted at |β| = 90° from the grating normal (i.e., it becomes an evanescent wave, passing over the grating horizon). For wavelengths λ < λ', |β| < 90°, so propagation is possible in order m' (and all lower orders), but for λ > λ' no propagation is possible in order m' (but it is still possible in lower orders). Thus, there is a discontinuity in the diffracted power vs. wave-length in order m' at wavelength λ, and the power that would diffract into this order for λ > λ' is redistributed among the other propagating orders. This causes abrupt changes in the power diffracted into these other orders.

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