T.E. Wellington

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 valueof the wavelength, lThe average of these runs is then compared to the_{m}.actual known wavelength, l_{k}

_{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

mis theorderof the pattern, one in most cases, l, is thewavelength,dis theslit 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,dis theslit spacing,xis thedistance between center and n=1 pattern, andlis thedistance from grating to source.

EXPERIMENTAL SET-UP

TYPICAL DATA

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