Finding the Value of Planck’s Constant – PRACTICAL – A Level Physics

Published on February 23, 2015 by

How to find the value of little 'h' yourself using some LEDs, a voltmeter and a variable power supply. Planck's constant is the constant of proportionality between the particle-like properties and the frequency of the associated wave.

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  • phezman 7 years ago

    Very helpful, thank you very much

  • jamal masarwa 6 years ago

    i keep doing this but it just wont give me plancks constant

    • Fisherdec 6 years ago

      +jamal masarwa From this, I’m thinking that you may need to look for the value of planck’s constant in electronvolts since he says E=eV.

  • jamal masarwa 6 years ago

    i have 550 nm gradient and 1.02v treshold

    • Killafuru 6 years ago

      +jamal masarwa Best way to calculate planck’s constant is if you have a Ek and frequency graph, the max kinetic energy Ek = q(electron charge)V and frequency which is f=c/wavelength, or at least that is what i used

  • Gail Glaister 6 years ago


  • Amin Shadowz 6 years ago

    By multiplying the gradient by the charge on the electron, do you just mean 1.6 x 10^-19?

    • Physics Online 6 years ago

      +Amin Shadowz Yes

    • Amin Shadowz 6 years ago

      +A Level Physics Online What’s the purpose of the potential divider circuit here? It doesn’t seem to be splitting the output voltage with another component. Thanks in advance

    • Physics Online 6 years ago

      +Amin Shadowz This allows you to vary the p.d across the LED.

  • Aarzoo Mir 6 years ago

    what exam board and spec are these videos built around?

    • Physics Online 6 years ago

      I tried to make them suitable for every spec, although I currently teach OCR Spec A.

  • Axel Rod 6 years ago


  • Glenn Ghose 6 years ago

    Its a really good experiment. I like the open circuit board where you can see all the components. Makes it less “black box” physics. Where did you buy the circuit board?

  • Mustafa Muhammad 6 years ago

    What pens are you using ?

    • Chandan Gupta 5 years ago

      Pilot Pens

    • Evelyn Lau 4 years ago

      Pilot v-sign pens

  • J D 6 years ago

    why does the blue light up later or in a higher voltage?

    • Hamza Ali 6 years ago

      it’s has a different threshold voltage

    • Abdelrahman Gamal 5 years ago

      Jasper D because blue light has a higher frequency and hence it requires more energy.

  • xVbM1 5 years ago

    What sort of graph would you get if you plotted the voltage to the wavelength instead of the reciprocal wavelength? Also is there a way of calculating planks constant without doing 1/wavelength and just doing wavelength?

    • Jonathan Fetzer 5 years ago

      I think it is because E or V = hf, so V/f = h and f = 1/wavelength.

    • Abdelrahman Gamal 5 years ago

      xVbM1 linear relationships are easier to work with. By taking the average of multiple readings, you can get more accurate results.

  • Jonathan Fetzer 5 years ago

    Awesome experiment!

  • St S 5 years ago

    First of all, your videos are great! Secondly, it would be really helpful if you make some videos on semiconductor physics including MOSFET and BJTs along with integrated circuits

  • Maria Hancock 5 years ago

    These explain the method very clearly, but it would be great to include some sources of error / limitations for each experiment, which would explain why our experimental value is never the same as the standard value.

  • BrUrai 5 years ago

    Nice and precise

  • Abdelrahman Gamal 5 years ago

    but this got me wondering.. why when we increase the voltage, the wavelength (the color) remains the same for the same LED !! When we increase the voltage, we actually increase the potential energy for each electron, and since E goes up, so should be the frequency! The color is supposed to change !!!

    • Griffin Kowash 4 years ago

      Light is emitted from the diode when an electron from the conduction band drops down into the valence band. In other words, when an electron transitions to a lower energy level, it releases energy as a photon. The energy of this photon (and by extension its wavelength) depends on the size of this gap between energy levels, and that depends on the type of material from which the semiconductor is made. Because the material remains the same, the wavelength remains the same as well.

      Increasing the voltage doesn’t change the wavelength, because it can’t change the chemical composition of the diode, but it does allow a larger number of electrons to make this transition per unit of time. This means that photons are emitted at a higher rate, which raises the intensity of the light, making it look brighter.

  • A G 4 years ago

    i know it is basic as f***, but why f=1/wavelength and not just f=wavelength?

    • Dennes Spek 4 years ago

      Hi, 1 stands for second. The amount of wavelengths per second is the frequency.

    • George Z 3 years ago

      So, it all comes down to the time variable again?

  • Roc Ren 4 years ago

    Sorry, but you do not explain how h is calculated.

    • Physics Online 4 years ago

      Have a look at 6:28 (using your value for the gradient and known values of e and c)

    • William Davis 3 years ago

      GCSE and A Level Physics Online it’s a bit of a cheat to require the value of e. How do you measure e?

    • Kris 3 years ago

      @William Davis It is a constant, just search it up online, there’s no need to measure it, but if you want to find out how it was measured search up: oil drop experiment

    • karolak kolo 3 years ago

      @Kris exactly. The video is about measuring h, not e. If you wanna be pedantic and do everything from scratch, just go repeat the oil drop experiment, albeit it’s much much harder to do (especially to do it precisely), and requires much much more calculations

  • Charles Barbour 4 years ago

    I find it strange that you go to the bother of making such a helpful video and some of the comments ask you to provide more information. WTF?!
    Thanks for taking the time to do this. It was extremely useful for me.

    • Nathaneal Afanu 3 years ago

      shut ur moute

  • Audemars 4 years ago

    Great video mate!

  • Isabelle Secord 4 years ago

    Thank you so much! This really helped me understand my required practical on Planck’s Constant.

  • Manasseh Nixon 4 years ago

    You sir are a legend, saving me from failing my as level physics exam! Thank you so much

  • Wolverine X 3 years ago

    Is this needed for A Level Edexcel Physics? Thanks!

  • Jeremy Ardley 3 years ago

    The apparatus is not well explained. Each LED in the kit has its own dropper resistor. They aren’t in parallel. If they were’nt separately fed and all the LEDs were in parallel, the red LED would clamp the voltage and the other LEDs would never light.

  • M 3 years ago

    Great video

  • 15past2 3 years ago

    The only problem here is that the wavelength of the LED’s, as given in the datasheets are calculated from planks constant. You are using circular logic!

  • Mohannad Islaieh 3 years ago

    Planck derived the value of his constant using his law of radiation

  • Deeve Sid Phillips 2 years ago

    So fast. Haha. It sucks to be such a slow learner. But maybe one day I will fully understand this. Very interested.

  • Pierre Retief 2 years ago

    I do have a question. If you know you will get a straight line, why not only do the test with the highest voltage LED, that should give you a V and lambda and you can use his last blue formula, what is the point of finding the gradient?

    • Physics Online 2 years ago

      To reduce an experimental errors and uncertainties – what if your result for the blue was significantly different, you wouldn’t know that was an anomaly if you only had one data point.

    • Pierre Retief 2 years ago

      @Physics Online Ahh ok, good point, thanks!

  • Sipan Hovsepyan 2 years ago

    Thanks a lot for an amazing video!

  • Chris_02 2 years ago

    How do you know the charge of the LEDs???!,???

  • Fabian B 2 years ago

    Would you get the same results if the lab where you do the experiment would be located on the ISS? What if the lab is located on the Moon or on Mars?

    • Physics Online 2 years ago

      You should do, it’s a universal constant so the same everywhere in the Universe.

  • a b 2 years ago

    Apologies, this is probably a dumb question, but with this experiment, are we making the assumption that the LED is 100% efficient?

    • Physics Online 2 years ago

      Yes, that’s right.

  • Shawn Grady 2 years ago

    Brilliant. Thank you. Simply stated and answered the question I had that several documentaries didn’t answer.

  • Yash Wani 2 years ago


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