T.E. Wellington 

Wavelength Determination of Laser Light Source

Based Upon Diffraction Pattern Analysis


A laser light source is directed horizontally towards a vertically mounted diffraction grating such that a pattern of peak intensities are distributed horizontally along a precision horizontally placed track at the appropriate level to be detected by a light probe attached to a cart. The position of the cart is continuously monitored for limited duration runs from one edge of the diffraction pattern to the other, with live graphs of Intensity versus Distance being produced using Logger Pro and a ULI interface.

A laser of known wavelength is used first, and the several runs used to establish a measured value of the wavelength, lm. The average of these runs is then compared to the actual known wavelength, lk

A laser of unknown wavelength is then used and an average value for its wavelength is established in the same manner as the known laser source procedure.



When light strikes a diffraction grating (a large number of equally spaced slits) the rays interfere with each other and a series of bright spots beginning in the center of the optical path and distributed to either side of the central spot appears.  It can be shown that the following formula relates the variables involved:
sin Q = ml/d


Where m is the orderof the pattern, one in most cases, l, is the wavelength, d is the slit spacing.  In fact, in this experiment, the ratio of distance between center and n=1 pattern to the distance from grating to source (tangentof angle Q) can be used instead of sine Q, since for the small angles used here sine=tangent to three significant figures.  Hence, the above equation becomes:

l = dx/l

Where l is the wavelength, d is the  slit spacing, x is the distance between center and n=1 pattern, and l is the distance from grating to source.