Notes for Experiment #9 -- Physical Optics
Notes & Hazards
Random helpful notes for your experiment:
- Notes on the Theory part:
Remember that waves interfere destructively when
they are out of phase (e.g., E+F=Z in Figure 1 on
p. 102); they interfere
constructively when they are in phase
(e.g., A+B=X in Figure 1 on p. 102). Remember, that in
order to have total destructive interference between 2
waves of equal amplitude and wavelength, they
must be 180o (or l/2) out of phase.
- Single Slit Diffraction
Diffraction is the bending of light around
corners. Diffraction effects occur for all kinds of
waves but are only apparent when the size of the object
is on the order of the wavelength. For today's
experiment, remember that the dark bands are
labelled consecutively for the single-slit (see diagram
below; the blue, rectangular boxes represent the actual
band of light you'll see, interspersed with the dark
spots representing destructive interference).
Notice that the dark bands are consecutively labelled
for single-slit diffraction and that y1, the
distance from the center of the central bright band to
the center of the first dark band, is simply
y1 - yo; similarly, the distance
to the 2nd dark band is just y2 -
To see how they get EQ5 on p. 104 (tan a = y/L), refer to the
Here, the tangent is simply the opposite/adjacent which
is equal to y/L.
- Double Slit Diffraction
Double slit diffraction is represented very well by
Fig. 4 on p. 105. Here, please remember that the
labelling of the dark and bright spots is intermixed
(see labelling of the y's in Fig. 4). The pattern
you'll actually see on the screen is illustrated below
(the blue boxes represent the light bands in the actual
spectra you'll most likely see):
- Notes on the Procedure part:
- The theory is really nice to understand but in order to
do the lab, you really only need to use 2 equations
(although it would be nice to understand them also,
wouldn't it?): use EQ 6 for the Single Slit part
(remember, n=1,2,3,... for the dark spots here) and
use EQ 13 for the Double Slit part (remember,
n=1,3,5,... for the dark spots here). Also, please keep
in mind that n is just the index for the y's; so
y1 has n=1, y2 has n=2, etc.
- Section 4.1, p. 108:
- Make sure you measure the fringes from the
center of the central bright spot
to the center of the dark area, as
- The metallic side of the slit (the one
without any labels) is the one you want to shine
- For Step 2: Keep your setup on the optical bench
so feel free to use a distance smaller than 1.5m
(as long as it's more than about 1m).
- Section 4.2, p. 108: For the double slit experiment, try
to shine the laser as directly as you can on the two
slits. This will help determine how good your
interference pattern is and hence how good your numbers
are. The double-slit interference setup assumes that
plane, monochromatic waves will be impacting both slits
equally; however, if the pen laser isn't oriented
exactly right, the laser beam will most likely strike
one of the slits more than another (and it might be
directed up or down, too). The slit that gets hit
preferentially will probably display a diffraction
pattern (since it's a single slit being hit by light,
it'll demonstrate the same kind of diffraction pattern
as you saw in Section 4.1 above). In this case, instead of having one
solid central block of light, you'll have a central
block that's made up of many smaller blocks (the
diffraction pattern from the single slit). The idea is
to orient the laser and the slit to minimize the
contribution from this extraneous, single-slit
diffraction. So if you see a whole lotta little blocks
within your big blocks, mess around with the orientation
of the laser and the slit until you've brought it down
to a minimum. Once done, you can calculate the y's as
- Section 4.3, p. 109: No experiment here; just simple
calculations. One partner can work on this while the
other sets up the next part (the only tricky thing about
Section 4.4 is the setup; the data-gathering itself is
quick and straight-forward).
- Section 4.4, p. 110:
- Be sure to unhook the resistor when you're
done with the experiment (this is to keep from
bleeding the battery).
- The photo-diode's input is on the opposite
side to the banana plug inputs and the battery.
- Start off with the initial position of the diode
assembly being 5cm from the screen. Also, make
sure everything's lined up to the same height
(i.e., the spot/dot created by the laser on the
screen, the photo-diode's input, and the pen
laser should all be at the same height).
- For this part, make sure you keep the laser and
the screen fixed BUT keep the photo-diode
assembly free to move so you can gather the data
at different distances.
- How can knowing the Voltages give us how many
photons there were per second (i.e., the
intensity)? Well, we know that Vlaser - Vno laser = DV = DEd = DIR and since I is equivalent
to the number of electrons flowing by per second
(and we know that 1 electron gets knocked out for
each photon that hits the diode), we essentially
get the intensity.
- Some good links:
- A really nice link that talks about solar cells,
interference, diffraction, etc.
- Minor errors only.
- Laboratory Manual (SGM 407)
- Laboratory Answer Book
- Calculator with statistical functions
Some Helpful Links & Miscellaneous Notes
- This lab should be relatively quick for most of us. The only
slow-down is sorta getting used to the equipment and knowing
which equation to use where. The above guidelines should help
keep you on track. Note: Section 4.5 is just a whole
lot of calculations so if you do some of those ahead of time
(or at least setup the equations), then you should get out of lab
in about 2 hours or so!
- Next week: Revenge of the Students, aka Teacher Evaluations!
Not to mention the last 135bL lab EVER!
- Feedback on if you're finding these pages helpful (or not) is
definitely good. So if you have any strong opinions, or, dare
I say it, ideas on how to make this better, drop me a line!
Also, if you have any thoughts on the midterm review, please
feel free to drop me a line (it'll only help improve the final
Ricky J. Sethi <firstname.lastname@example.org>
Last modified: Mon Apr 17 15:28:19 2000