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This javascript app calculates the quantity and angles of diffracted orders of monochromatic light when shone on a reflective diffraction grating. Orders up to m=10 are all shown graphically and numerical values are given above the graph. See below for a key and equations.
Enter an angle for alpha between 0 and 90°. All angles are between their line and the grating normal, the dotted vertical line. Angles to the left of the grating normal are positive and those to the right of the line are negative.

α = °     rulings =      λ = nm    

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Key:
Green: The incident ray. The angle between this and the grating normal is alpha (α).
Orange: This is the reflected ray, beta (β). The angle between it and the grating normal is the negative of alpha.
Brown: These are the diffracted rays. The javascript app will display all positive and negative orders up to and including m=10.

Equations:
The angle of each order from the grating normal (GN) can be calculated using the following equation:

Where:
   βm is the angle from the grating normal to the mth order
   m is the order
   λ is the wavelength in meters
   d is the distance from the center of one slit to the next
   α is the angle of incidence

Copyright James Williamson 2012