Nature is rarely as simple as physicists describe it. Consider Ohm' s law -- voltage is proportional to current. This is exactly true only for "Ohmic" materials, which are defined as materials for which voltage is proportional to current. Same goes for the Ideal Gas Law, which is strictly true only for ideal gases, which we can define as gases for which the Ideal Gas Law holds.

Let us consider a non-Ohmic object: the diode. You will learn about the theory behind the diode and understand why it works in class, probably weeks from now. Today what we want to study is the behavior of current vs voltage for the diode.

(See also, for your own amusement, The Light-emitting Vegetable Diode.)

PRELIMINARY. Measure the resistance of an LED with an ohmmeter. Measure it again with wires reversed. Record your readings.

LED PLUS OSCILLATOR PLUS OSCILLOSCOPE. Connect an LED to an AC oscillator. Put an alternating voltage across the LED and display the time dependence of the voltage across the LED on an oscilloscope. Why is the voltage not a constant? Sketch V(t) in your notes, marking the axes with the appropriate units and scales.

1. What voltage is necessary to "turn on" the LED?
2. At what frequency of oscillation do you lose your ability to notice the flicker of the diode turning off and on? (This is a physiology/psychology question, but it's still an interesting experimental question)

MEASURING CURRENT. What' s missing is an idea of the current flow in the circuit. We can get this by connecting the diode in series with a resistor. The voltage across the resistor equals V=IR, so, by displaying this voltage on the scope, we can see how the current varies with time. Choose a resistor comparable to the impedance of the oscillator, and put it in series with the diode. Connect the point at which diode and resistor touch to the ground of the scope. Connect the point on the other side of the diode with the "horizontal" channel of your scope, connect the point on the other side of the resistor with the "vertical". Sketch V(t) and I(t) in your notes.

3. What is the resistance and power dissipation in the diode when it is on at full brightness?
4. Compare, qualitatively, the resistance of the diode for positive and negative current.

PLOTTING CURRENT VS VOLTAGE. Now use the XY display mode of your oscilloscope to display the I vs V curve for the diode. Be sure to determine where on the scope the origin (V=0=I) is.

A better way is to simply use a "Curve Tracer", which is pretty much the same circuit as we have been using, except more "user-friendly". Time permitting, insert your LED in the curve tracer and try to reproduce the I(V) plot you created above. Notice that you need to flip the Polarity switch in order to see both halves of the curve. 5. What voltage is required to produce significant current?

5. Record this voltage for blue, green, and red LEDs. How do these voltages compare to the energy (in eV) of a typical blue, green, or red photon?


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 (BEFORE lab). How do they work?

CIRCUITRY. If we think of the photodiode as a device with a resistance which depends on light intensity, and if the resistance is inversely 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 x 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 Vout? Connect such a circuit, using a prototyping breadboard and an IC socket, so that you can easily change resistors.

LINEARITY. We will use a neutral density filter to decrease the light hitting a photodiode, and calculate wether its response is linear, that is, whether decreasing the light hitting it by a factor of ten decreases its signal by a factor of ten. The transmission of a filter of density, D=1.00 is 10%, of a filter of D=2.00 is 1%, and so on. That is, its transmission is given by T=10-D. Check the linearity of your photodiode over at least three orders of magnitude.

1. Compare the AC and DC voltages registered by your photodiode sitting in the lab room. Explain why there are both types of signals, and use an oscilloscope to find the frequency of the AC component.
2. Compare the AC and DC voltages registered by your photodiode in a darkroom with a tungsten-filament bulb. Explain.

Soap suds: Take a test tube, construct a holder for it, put a little detergent and some water in the test tube, and make a rich lather 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 bubble grows with time.

NEXT 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 you want to do.

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