What it shows
An LED only begins to conduct above a threshold voltage that is higher for colours of shorter wavelength: red about 1.8 V, green about 2.1 V, blue about 2.8 V for typical 5 mm LEDs. The threshold energy e V roughly equals the photon energy h c / lambda, so this experiment gives an estimate of Planck’s constant.
The science
An LED is a p-n junction in which electrons falling across the band gap release their energy as a photon. The band gap energy sets both the colour of the light and the voltage needed before current flows: a bigger gap means a bluer photon and a higher turn-on voltage. Measuring the voltage at which each LED just starts to glow and multiplying by the electron charge gives the photon energy e V, and setting this equal to h c / lambda with the wavelength from the LED packet gives h to within roughly 10 to 30 percent. The V-I curve of an LED is not a straight line, which also shows it is a non-ohmic device.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Red, yellow, green, blue and white 5 mm LEDs (about Rs 2 each)
- A 10 kilo-ohm potentiometer (variable resistor)
- Three AA cells in a holder (4.5 V)
- A 220 ohm resistor
- A digital multimeter
- A breadboard or crocodile clips, and graph paper
Safety
For this project: Keep the 220 ohm resistor in series always, or the LED burns out. Never look straight into the blue or white LED at full brightness. Cells only.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Wire the cells, the potentiometer used as a variable resistor (centre pin joined to one end pin), the 220 ohm resistor and the red LED in series, with the meter across the LED on DC volts.
Start with the potentiometer at its highest resistance and lower it slowly in a darkened room until the LED just visibly glows; record this turn-on voltage across the LED.
Continue lowering the resistance in small steps and record the LED voltage and the current (meter in series on milliamps) at each step, up to about 10 mA.
Repeat the turn-on voltage and the V-I table for the yellow, green, blue and white LEDs.
Record a results table with columns for colour, wavelength (from the packet, or typical values: red about 630 nm, yellow 590, green 525, blue 465), turn-on voltage, and e V in electron volts.
Plot the V-I curves of all LEDs on one sheet to show the sharp knee and the different thresholds, and extend the straight part of each curve back to zero current to get a second, less subjective turn-on voltage.
Plot turn-on voltage against 1 / lambda for the single-colour LEDs (leave out the white one, a blue chip with a yellow coating that has no single wavelength); the slope multiplied by e / c gives an estimate of Planck’s constant.
Compare with the accepted value 6.63 x 10^-34 J s and explain the sources of error, such as judging when the glow starts.
What you should see
Turn-on voltages rise from red to blue, V-I curves show a sharp knee, and the Planck constant estimate comes out within roughly 30 percent of the textbook value.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why does a blue LED need more voltage than a red one?
Blue photons have more energy, so the semiconductor band gap and the turn-on voltage are larger.
02Is an LED an ohmic device?
No; its V-I graph is a curve with a knee, not a straight line through the origin.
03Why is the series resistor needed?
Above the knee the current rises very steeply with voltage, so the resistor limits it to a safe value.
04What relation links the photon energy and wavelength?
E = h c / lambda.
Take it further
Measure the LED voltage in the dark with a bright torch shining on it to show the small photovoltaic effect in reverse.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Investigatory Projects for Class 11 and 12
Topics, method and report structure for physics, chemistry and biology
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each
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