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Double Slit Interference
Objective
The objective of this lab is to study the wave nature of light by
performing Young's double slit experiment.
Background
Thomas Young's double slit interference experiment, first done in
1800, provided crucial evidence for the wave nature of light.
The interference of sound waves is easily heard, when two sound
sources are generating the same frequency of sound, alternating
positions of constructive and destructive interference is
observed. The interference of light waves is not so easily seen,
the interference pattern is observed when the slit spacing is
small and the incident light is coherent.
When coherent light, the light waves are in phase with each other, is incident upon two slits the resulting pattern on the screen consists of alternating bright and dark bands. Coherent light can be obtained by coordinating the radiating atoms so they are always in phase, this is done in a laser.
With the laser beam centered on the double slit, if light were not a wave the screen would be totally dark. Instead, each slit acts as a source of waves, recall Huygen's principle. Where these waves interfere constructively, a bright band is observed, where these waves interfere destructively, a dark band occurs. The interference pattern is a series of alternating bright and dark bands or fringes.
When the path difference (BC) is equal to one wavelength or some integral number of wavelengths, the waves are in phase and constructive interference occurs. Otherwise destructive (or partially destructive) interference occurs.
When the path difference (BC) is an integral number of
wavelengths (ml), similar triangles
shown in the above diagram can be used to show that: 
Where ym is the position of the mth bright band with respect to the axis, L is the distance from the slit to the screen, d is the slit spacing, and l is the wavelength of light being used.
Equipment
- optical bench
- laser
- double slit
- screen
- lamp
Procedure 
1. Remove the slide holder from the optical
bench and examine the four sets of double slits. Record the slit
spacing for the first two sets of slits (A, B). Return the slide
holder to the optical bench.
2. Turn on the laser and adjust the height of
the beam such that it strikes the middle of the slide and is
directed to the center point of the two slits marked A, that is,
center the laser beam so that an equal amount of light passes
through each of the two slits.
3. The double slit interference pattern should
now be observed on the wall/screen. Make any further adjustments
to the laser position or the slide position to get the bands as
sharp as possible. Measure the distance from the slit slide to
the wall.
4. Tape a piece of paper to the wall or screen
to record the pattern. Carefully mark the position of the central
maximum (middle bright band) and each of the bright bands to both
sides of the zeroth maximum.
5. Remove your paper from the wall and turn off the laser.
Analysis
1. Carefully, accurately measure the distances
(s1, s2, s3, .....)
2. Calculate the average (y) values for each set
of bright bands on either side of the central max.
y1 = (1/2)s1 , y2 = (1/2)s2
, ... etc.
3. Determine the wavelength of the laser light
using the bright band equation for each average (y) value
obtained. Determine the average of those wavelengths.
Repeat the Procedure and Analysis for the second set of slits marked as B.
Related Activities
Ask the instructor for the wavelength of the laser light being
used and compare that value with your measured value.