Download LED - UCSD Jacobs School of Engineering

Light Emitting Diode (LED)
Experiments
A. Observing Diode Behavior
Directions:
1. Connect your LED Color Strip to the battery. What happens?
2. Turn the battery around, so that the electrical contacts are reversed, and repeat the experiment.
Only make this reverse connection momentarily, as damage may occur to the capacitor if the
reverse connection is left in place. (Note: you cannot connect the battery in reverse, but you can
touch the terminals.) What happens now?
3. Touch the leads of the small incandescent bulb to the positive and negative terminals of the 9V
battery. What happens to the bulb? Now turn the light bulb around so the electrical contacts are
reversed. Make note of what you see.
Questions:
1. What happened when the battery was reversed and placed in contact with the LED Color Strip?
Did the incandescent light bulb behave similarly to the LED Color Strip? Describe any
differences you noticed.
2. What causes the incandescent bulb and the LEDs to behave differently when their electrical
connections to the battery are reversed?
Determining the Relative Wavelengths of Light
Directions:
Position the LED Color Strip vertically by hanging the Strip over the edge of the observation
screen. Stand approximately 1 meter away from your LED Color Strip, place the diffraction slide
up to your eye, and view the light source through the diffraction grating. Draw what you see. If
the streaks of color are above and below the LED Color Strip, rotate the diffraction grating 90°
so that the streaks appear to the side.
Questions:
1. For which color light does the diffraction streak appear farthest from the LED Color Strip? For
which color light does the streak appear closest to the LED Color Strip? Which LED has the
longest wavelength? The shortest?
2. Examine an incandescent or fluorescent bulb with the diffraction grating. Does either give just
one color of light? Do the colors you see come in the same order as when they are produced by
separate LEDs?
Determining the Wavelengths of Light Quantitatively
Directions:
1. Attach the LED Color Strip to the battery and hang or clamp the LED Color Strip over the
edge of the observation screen.
2. The diffraction grating should be 50-100 cm directly in front of the LED Color Strip. Measure
and record the distance L between the diffraction grating and LED Color Strip. Keep this
distance constant throughout the experiment.
3. Look through the diffraction grating (your eye should be very close to the grating without
touching it) and make sure you can see the streaks of color off to the sides.
4. Use a white piece of paper to serve as a marker (or make marks directly on the observation
screen). Have your partner hold the paper. Direct your partner to move the paper until its edge
coincides with the brightest area of the streak that corresponds to the red LED. Measure the
distance x, of the marker from the LED color strip.
5. Repeat step 4 for each color LED.
6. Calculate the diffraction angle θ for each LED using the formula tan(θ) = x/L.
7. If period of the diffraction grating, d = 2µm (based on 500lp/mm), calculate the wavelength
for each LED using formula: d sin(θ) = λ .
Question:
Do the wavelengths you calculate follow the same trend as the relative wavelengths you
determined in the previous experiment?
Determining the Relative Energies of Colors Using a Capacitor
Directions:
1. Connect the LED Color Strip to the battery.
2. As you disconnect the battery, watch the LEDs. Note your observations.
Questions:
1. Which LED fades first? Which stays lit the longest?
2. What can you conclude about the voltage required for each LED?
3. Based on what you observe after the battery is removed, how do you think the capacitor
voltage varies with time?
Measuring the Voltage of LEDs Using a Voltmeter
Directions:
1. Turn on the power to your voltmeter. If you have a multimeter (one that measures volts,
amperes, and ohms, for example), make sure that you have selected "volts DC". This is
sometimes abbreviated VDC, or V followed by a solid horizontal line above three dots. The
LEDs will all have voltages between 1 and 4 volts, so choose the range accordingly. Also, make
sure one lead is plugged into the "com"(or "-") side of the voltmeter and the other is plugged into
the hole labeled "V" (or "+").
2. Your LED Color Strip has a post next to each LED, plus another labeled "com". Connect the 9
volt battery to the LED Color Strip to activate the LEDs. Attach the lead that is plugged into
"com" on the voltmeter to the post labeled "com" on the LED Color Strip. Touch the "V" lead
from the voltmeter to the post next to the blue LED. The voltmeter will read the approximate
voltage that this LED uses. Keeping the "com" lead attached, repeat this measurement for the
green, yellow, orange, red, and infrared LEDs. Record your observations.
Questions:
1. Which LED has the highest voltage? Which has the lowest? Note that even though you cannot
see the light emitted by the infrared LED, you can still measure the voltage it uses, and hence
estimate the energy of the light it emits.
2. Make a table of the voltages you measured. Do these data support the conclusions you reached
in the previous experiment about the voltage required for each LED?