To find the wavelength of the light, to find  slit separation for the double slit experiment.

 Laser light source, screen, slides with different slits.

Diffraction is the property of all waves to bend around obstacles. This is the wave property that allows one man to hear another speaking when one is "around the corner" of a building from the other. Geometrical optics predicts that light rays follow an undeviated straight line in a uniform medium. Shadows of objects should be distinct, with clear sharp edges, if the pure geometrical description is correct. Careful examination of an optical shadow reveals that the "shadow" has a series of "edges", instead of a unique edge. This is the diffraction effect in light.

The diffraction effect in light is usually accompanied by interference effects similar to those observed in the Michelson interferometer experiment. In fact a good example of these two effects can be seen by passing parallel laser light through a small diameter hole and observing the shadow of the transmitted light. The light will be circular, with a diameter much larger than the original laser light. Around this central disc of light will be found a series of concentric rings of light, due to interference between rays of light diffracted at different positions across the opening of the hole.

The general equation for diffraction from slits is      n l = d sin Q                                                                         (1)

where n is the order number, l is the wavelength, d is the appropriate slit spacing or width, and Q is the angular deviation from the original direction of the laser beam to either the diffraction minimum (single slit) or maximum (multiple slits).
 

Exercise 1.

Introduce a small aperture into the laser beam, either a circular hole or a slit. Observe the diffraction pattern on a screen with its bright, wide central maximum, centered where the undeviated laser beam would have been. Note the symmetrically distributed bright and dark regions (maxima and minima, respectively).

Exercise 2.

For a given single slit aperture (rectangular shape), measure the distance from the center of the central diffraction maximum (observed on a screen) to the series of diffraction minima to one side of the central maximum. Measure them in order and number them starting with the minimum closest to the central maximum (let it be the first order) and increasing the order number consecutively as the distance to each minimum increases. Repeat these measurements for at least five successive minima and three different slit widths. Be certain to record the aperture to screen distance and the aperture width.

Exercise 3.

Introduce a double slit into the laser light and record the location, from, the central maximum, of all the maxima and minima by measuring the distances as in Exercise 2. Be certain to record the slit widths, the distance between the two slits, and the screen to aperture distance. Select a double slit that allows you to measure ten to fifteen light intensity maxima.

Exercise 2.

Determine the wavelength of light using equation (1). Graph of the sine of the angle (to the diffraction minima) as a function of the order number. You should get a straight line with a slope of l/d. Each single slit used will provide another graph and another measurement of the wavelength of the laser light. Obtain a statistically weighted average of these wavelengths, using the calculated wavelength errors as the statistical weights.

Exercise 3.

Using the wavelength of light determined from the exercise 2, calculate the "slit width", d. Make a graph of the sine of the diffraction angle (to the maximum) as a function of the order number.  You should get a straight line with a slope of l/d.  Calculated  d  from the slope. How does this compare to the value of d given on the double slit slide? If your straight line appears to be broken segments of parallel lines, you may not have corrected properly for the missing orders. At what diffraction angles do you expect to observe missing orders for your slit widths?