Can I use fluorescent light bulbs to calculate Planck's constant? We did an experiment today to calculate Planck's constant, $h$. We measured the knee voltages $V$ of different LEDs (red, green, orange, blue, violet) and plotted them against the frequencies $f$ of these lights. The equation of the line is given by $V = af + b$. where $a$ is the slope of the graph and this numerical constant was approximated as Planck's constant. 
I'm just curious if we could redo this experiment but instead of LEDs, we use fluorescent and incandescent bulbs. Would we still be able to get Planck's constant? 
 A: No. That equation is only valid for the photoelectric effect. Since fluorescent lights use the concept of mercury spectrum emission (the emitted UV rays cause fluorescence on the tube's phosphor coating) and incandescent bulbs use the principle of thermal emission by Joule heating, the said equation won't work.
Even then, considering that these incandescent lamps are radiating black bodies, we could use Planck's emission spectrum equation or Wien's displacement law to calculate Planck's constant. However, it wouldn't be as simple as measuring the slope of a graph.
However, the LEDs used do operate on the reverse concept of the photoelectric effect (electron-stimulated photon emission), so the equation is applicable for them.
A: According to http://www.scienceinschool.org/2014/issue28/planck, All you need to calculate Planck's constant is 
Four LEDs emitting coloured light – one each of red, orange, green and blue. Choose LEDs w

  
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*Four LEDs emitting coloured light – one each of red, orange, green and blue. Choose LEDs with a clear, colourless casing surrounding the LED, so that the colour of the light comes from the device itself, not from the coloured casing. 
  
*9 V battery. 
  
*Two multimeters (one to be used as a voltmeter and the other as an ammeter). 
  
*1 kΩ potentiometer or rheostat.
  

If you follow the procedure listed on the site above, you will be able to calculate Planck's constant with a percent error of around only 0.7%.
Given this information I can address the question by saying no, you cannot use fluorescent light bulbs to calculate Planck's constant but yes, you can use LEDs to approximate Planck's constant with a margin of error smaller than one percent. I hope this helps.
