Physics IA · E · Nuclear and quantum physics · topic 26 of 30
LED colours: can you measure Planck's constant? · IB Physics IA idea: guide, sample graph and step-by-step plan
a quantum result from a 20p component :)A tiny LED can give you Planck's constant. But when does it just glow, and how sure are you?
- ✓ you like linking circuits to quantum ideas
- ✓ you can judge a faint glow in a dark room
- ✓ you enjoy deciding what counts as "on"
The big idea
← 'threshold' is the whole problemAn LED emits a photon when an electron crosses its band gap, so the voltage at which it just lights should give the photon energy: eV ≈ hc/λ. A graph of threshold voltage against 1/λ should be a straight line with gradient hc/e, and from it you can find h. The hard part is the word threshold: you can judge the first glow by eye, or find it from the current–voltage curve. Which method you choose, and how well you defend it, makes the investigation.
Make it yours: pick one twist
Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.
Swipe for more twists →
Record current against voltage for each LED and extrapolate the straight part back to the voltage axis.
→ Replaces eyeballing with a measured, repeatable threshold and an uncertainty from the fit.Judge by eye in the dark, and by extrapolating the I–V curve.
→ Two values of h with uncertainties. The evaluation is about which method is more reliable, and why.Use a diffraction grating, a spectrometer app or a school spectrometer rather than the datasheet.
→ Wavelength becomes a measured quantity with its own uncertainty, including the width of the LED's spectrum.Repeat for one LED at several temperatures, with the LED sealed from water.
→ The threshold shifts with temperature. A second, smaller effect; it needs good control.Use a phone camera to see an infrared LED and find its threshold.
→ Extends the range of wavelengths, so the line is better determined, but needs a way to measure λ.Ask why real LEDs give a value of h with an offset, and test an alternative model.
→ A deeper analysis of the intercept: the theory predicts zero, the data may not.Your Physics IA research question: run the check
write it FIRST, then check itWrite your own question first. Then make sure it ticks every box:
- Names the independent variable (LED colour, as a wavelength) and how many LEDs you will use.
- Names the dependent variable and says exactly how you decide the threshold voltage.
- Says what you will keep constant: the temperature, the circuit and the room's darkness.
- Says what you will compare with (the accepted value of Planck's constant, or the value from a second method).
- Is narrow enough for about 10 hours, with at least 5 colours and repeated threshold readings.
The set-up
Raise the voltage slowly and note where the LED just glows, or record current against voltage. Draw your own circuit with real values.
How your Physics IA could be laid out
The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.
Your question, why the link between light and energy matters (the photon model, LED lighting) and what you expect.
Photon energy, the band-gap idea at a level you can explain, and why V against 1/λ should be a line.
Independent, dependent and controlled variables, with a definition of "threshold".
The circuit and why the resistor is there, how you decide the threshold, how you find each wavelength, and how many repeats.
Maximum LED current, a safe voltage, not looking at a bright LED from close up.
Threshold voltages and wavelengths with units, resolution and repeats.
Uncertainty in V and in 1/λ, and one worked example of each calculation.
V against 1/λ with error bars in both directions, best and worst lines and what the gradient and intercept mean.
Your value of h with uncertainty, compared with the accepted value; whether the intercept supports the model.
Weaknesses ranked by effect on h, linked to specific improvements.
Sources for the theory, the accepted constants and the LED data.
What your Physics IA graph could look like
what does the intercept mean?Made-up data, to show the form of a strong graph, not the answer. Yours will look different. example only
- Using 1/λ rather than λ makes the relationship a straight line, so you can fit it and read a gradient.
- Error bars both ways: the voltage from judging the threshold, the 1/λ from the spread of colours in one LED.
- Draw the theory line through the origin, then your own best and worst lines. Where do they differ?
- Think about what the vertical scale does: it starts away from zero, so the intercept is not shown. Say so.
- Another graph to consider: the I–V curves of all LEDs on one set of axes, to show how you extracted each threshold.
Which uncertainty matters most?
Rank by the effect on your result, here the value you get from one blue LED. Notice that the biggest items are about judgement and the light itself, not the meter.
Your game plan
define 'just glows' in the trial runTick steps off as you go. We'll remember on this device.
0 of 10 done
- Read and predictRead about photon energy and LED emission. Predict the shape of the V against 1/λ graph and what the gradient should be.
- Trial run (1 lesson)Take one LED and a few others. Raise the voltage slowly in a dark room: can you agree with yourself, and with a friend, about when it "just" glows? Try recording the I–V curve too.
- Decide how to define the thresholdChoose a method and write down exactly how you apply it every time. Keep the room and the viewing distance the same.
- Find the wavelengthsDecide whether you use datasheet values or measure them. If you measure them, plan how you handle the spread of one LED's spectrum.
- Collect the dataTake repeated threshold readings for each LED, with the series resistor in place. Record temperature and any odd behaviour.
- ProcessTurn wavelengths into 1/λ with uncertainties, and decide how to show both uncertainties on the graph.
- Graph and fitPlot V against 1/λ, add error bars, draw best and worst lines, and find the gradient and its uncertainty.
- Compare with theoryConvert the gradient to h and compare with the accepted value. What does the intercept say?
- EvaluateRank the uncertainties by effect on h and link every improvement to a specific weakness.
- Check before your teacher's draftRun the checklist above, then get your RQ or draft checked. Your teacher reads one draft only.
Physics you'll need
Traps to avoid in this Physics IA
- "Just glows" judged differently each time: write down your rule and keep to it.
- No series resistor, so the LED is damaged or the current is far too high.
- Taking the colour name or a rounded wavelength instead of the actual peak.
- Ignoring the width of an LED's spectrum when giving the uncertainty in λ.
- Using one LED per colour with no repeat, so you cannot tell a real trend from a faulty part.
- Quoting h to many significant figures with no uncertainty.
More Physics IA ideas like this
- Photoelectric effect: stopping voltage vs frequency●●○
- Dice decay: does chance follow the exponential law?●○○
- Counting radiation: does it follow an inverse square?●●●
This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.