Photodiodes are one of the most widely used transducers
for measuring the intensity of laser light. You will design and build a
circuit which produces a signal from a photodiode which is linear in intensity,
you will test the linearity, and then you will use this circuit to perform
an unrelated experiment.
LIBRARY WORK. Find out what you can about the physics
of photodiodes. How do they work? For what wavelengths would you expect
a silicon diode to work? Germanium?
CIRCUITRY. If we think of the photodiode as a device with
a resistance which depends on light intensity, and if the resistance is
proportional to the intensity, how would we design a circuit to get a signal
that is proportional to intensity? (not by measuring the resistance)
If we put a voltage across the diode and measure the current flow, would
that do it? Put a resistor, R, in series with the diode, and put
a voltage, E , across this
series. The resistance of the photodiode equals r=A/x, where
is the intensity of the incident light. Check whether the voltage across
either r or R is nearly linear in x. What are the
limits of linearity, in terms of r and R? In terms of
Connect such a circuit, using a prototyping breadboard and an IC socket,
so that you can easily change resistors.
LINEARITY. The light reflected off a glass slide at normal
incidence is given by Fresnel's equation,
with n=1.50. Reflect a laser off one, then two slides,
and compare the measured intensity (from your diode) to the original beam.
Is your transducer linear? What do you calculate for n?
ELECTIVES: Do #1, plus do either #4, or both #2
and 3. 1Ù (4Ú
1. Compare the AC and DC voltages registered by your transducer
sitting in the lab room. Explain why there are both types of signals. (Use
an oscilloscope to find the frequency of the AC component.)
2. Compare the AC and DC voltages registered by your transducer
in a darkroom with a tungsten-filament bulb. Explain.
3. Verify the inverse-square law for a standard light
source (no lasers!).
4. Soap suds: Take a test tube, construct a holder for
it, put a little detergent and some water in the test tube, and produce
a rich foam of bubbles. Shine the laser through the foam. See whether it
is possible to measure any output signal through the foam. Calculate the
number of bubble surfaces, and thus the mean size of the bubbles, from
the output intensity, given Fresnel's equation, with n=1.33 (water).
Two experiments you might want to conduct with this system
are the following: (a) After the suds have settled a little, measure the
unreflected light through the test tube as a function of height (Correlate
this to what you measure directly for the size of the bubbles.), or (b)
Leave the system stationary and measure the unreflected light at a fixed
height as a function of time. This second experiment will take much longer,
but it may tell you how the average size of the bubbles evolves with time.
We will do independent projects, so sit down and have
a chat with your lab instructor well in advance of then to figure out what
project to do.
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