Measuring Plank's constant with LED's

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Discussion Overview

The discussion revolves around an experiment aimed at measuring Planck's constant using LEDs of various colors. Participants explore the calculations involved, the implications of voltage and wavelength, and the relationship between energy, frequency, and wavelength in different media.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant reports measuring Planck's constant as approximately 3.7 x 10^-34 Js using a red LED, questioning the accuracy of their result and potential errors in their calculations.
  • Another participant asserts that the speed of light used in calculations should be the speed in vacuum or air, suggesting that the energy from a 1 eV electron cannot produce a 700 nm photon, indicating a possible misunderstanding of the LED's voltage or operation.
  • A participant expresses uncertainty about the visibility of light at 800 nm and the implications of using different voltages for the red LED.
  • Another participant clarifies that the fundamental relationship involves energy and frequency, noting that while frequency remains constant, wavelength measurements depend on the medium's speed of light.
  • Concerns are raised about the color of light being dependent on frequency or wavelength, leading to confusion regarding measurements in different materials.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the accuracy of the measurements or the calculations involved. Multiple competing views regarding the relationship between voltage, wavelength, and the visibility of light remain unresolved.

Contextual Notes

Participants express uncertainty regarding the correct application of physical principles, such as the speed of light in different media and the energy requirements for photon emission from LEDs. There are also unresolved questions about the visibility of light at certain wavelengths.

resurgance2001
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I have been trying to do an experiment measuring Plank's constant using LED's of different colors. The result I get is out by a factor of nearly two. I.e. I got about 3.7 x 10^-34 Js . I am not sure what I am doing wrong.

Using a red Led for example, which has a maximum wavelength of about 700nm, and a turn on voltage of about 1 volt, I do the following calculation. E = hf. So h = E/f = (1 x 1.6^-19 )/ (3 x 10^8/700 x10^-9) = 3.73 x 10^-34.

Should I be using a different value for the speed of light because the light is being first emitted in plastic? Clear plastics typically have refractive indexes close that of glass. Using a value of 2 x 10^8 for plastic still only Πgives planks constant as 5.5 x 10^-34 Js. This leads me to thinking that I have made some other fundamental blunder, but I am not sure what it is. Should the be a factor of pi in there or something?
 
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The speed of light is the speed of light in vacuum (or air, does not matter), as you measure the wavelength there.

Using a red Led for example, which has a maximum wavelength of about 700nm, and a turn on voltage of about 1 volt
That looks suspicious. An electron with an energy of 1eV cannot produce a 700nm-photon. Either there are 2-electron processes happening of something else is wrong.
Are you sure you get light with a voltage of just 1V?
 
Thanks. I will check the voltage again more carefully. But even with 1.5 volts for a red LED, the wavelength at the shotch on sholder would have to be 800nm. Are we able to see 800nm?
 
Just looked agian at your answer mfb. Why does the speed have to be where I measure the wavelength? The colour of the light stays the same in glass or plastic doesn't it? Is the color of light dependent on the frequency or wavelength? I am confused..
 
The fundamental relation (via the Planck constant) is between energy and frequency. The frequency is the same everywhere, but hard to measure. If you measure the wavelength in some medium (or vacuum), you have to use the local speed of light to convert it to the right frequency.

800nm is somewhere at the edge of the visibility - intense laser light can be visible, seeing an LED would probably require a very dark room and a bright LED.
 

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