Interference of light wave 4

Objective
The objective of this experiment is to investigate double-slit interference of light. You will exploit
the relationship among slit spacing, wavelength, and fringe spacing to measure the wavelength of
light starting with only a ruler as a standard of length. The investigation will also illustrate how a
well-characterized slit pair can be used to determine the wavelength of an unknown light source.
The procedure illustrates how averaging of multiple measurements can increase the precision of a
measurement.
Procedures
1. Prepare a slit pair.
 Collect a thin lm of carbon black on part of a microscope slide by holding the slide in the
orange part of a candle
ame. Above the
ame, the combustion gases contain mostly H2O
and CO2, but inside the orange part of the
ame, combustion is incomplete and there are
many carbon atoms available to precipitate onto the relatively cool glass of the slide. While
blackening the slide, keep it moving so as not to overheat it, and deposit just enough carbon
to make the slide opaque. You want as thin a lm as you can get while making the slide
nearly opaque.
 Scribe a double slit in the carbon lm with a pair of razor blades pinched tightly together.
Use the corners of the two blades and be sure that both corners are in contact with the glass
before making a quick stroke across the glass.
P4: Young s Experiment Rev 4 YJB 8May15 p. 2 / 5
 You may have to try this procedure several times before you get a clean slit pair. You can
make several slit pairs on the slide and then choose your best one. If you need to start over,
just wipe the soot o the slide with a paper towel.
2. Measure the double-slit interference pattern with a red laser.
 Place the red laser pointer and your slide into holders to steady them. Arrange the apparatus
so that the laser shines through the slits and onto a screen a few meters away. You should
be able to use the screen provided on your lab bench, but you might nd it interesting to
project onto a distant wall.
 Measuring the spacing between a single pair of fringes will be inaccurate, but if you measure
the distance covered by many fringes and then divide by the number of fringes you can make a
more accurate measurement of the basic fringe spacing. By measuring the spacing of dierent
fringes many times and averaging, you can reduce your error even further. If you are really
ambitious, you could repeat the measurement for more than one screen range.
 Make a data table with columns for measured screen range, number of fringes counted, the
measured width of the counted fringe array and the computed fringe spacing. Be careful
about what you observe here. There are really two patterns of fringes on the screen: there s
a single-slit pattern due to the width of you slits, and there s a double-slit pattern due to the
separation of the slits. You want the double-slit pattern; it s the smaller of the two. Check
your measured slit spacing against the estimate you prepared for the pre-lab to convince
yourself you are measuring the right thing.
At this point it should be clear that you do not have enough information to determine either the
wavelength of the laser or the slit spacing. To nd the slit spacing, you need a laser of known
wavelength. The next few steps describe a procedure for calibrating the wavelength of the green
laser. You will then use the green laser to measure the slit separation. With a known slit separation
you can nd the wavelength of the red laser.
The idea is to use a short piece of wire of known diameter to create an interference pattern similar
to the double-slit pattern. A wire placed across a laser beam in place of the slit pair will also
produce an interference pattern. Light passing around the two sides of the wire will interfere with
each other just as light passing around the two sides of the barrier between the two slits. But
superimposed on top of that double-slit pattern will be the main beam plus an interference pattern
due to light farther from the wire. The wire interference pattern may be somewhat more dicult
to interpret, but the primary fringe spacing will obey the same law as the double slit.