# Finding Planck's Constant with LEDs

• Paul37
In summary, to find Planck's constant using LEDs, one must first measure the LED's wavelength and frequency using a spectrometer. Then, using the equation E=hf, the energy of the LED can be calculated. To find this energy, the voltage that the circuit eventually tends to should be used. Additionally, the energy of a UV LED can be measured by placing it in a photovoltaic cell.
Paul37

## Homework Statement

I am trying to prove Planck's constant using LEDs. So far I have measured several LED's wavelengths using a spectrometer and from this their frequency. This will be f in the equation h = E/f. For E I put the LEDs in a forward biased circuit and am trying to use the equation E = qV to find E so I can therefore find Planck's constant. My question is which value do I use for V? The minimum voltage needed to produce a current in the circuit or the voltage the circuit eventually tends to.

## Homework Equations

E=hf
E=qV

Also, do you know of any related experiments that I could do concerning this sort of area. I was going to find the wavelength of a UV LED and find its energy by a different method (not going to look at a UV LED through a spectrometer) but I wasn't sure exactly how to do that.

The Attempt at a SolutionFor the first question, you should use the voltage that the circuit eventually tends to. This is because the voltage that is needed to produce a current in the circuit will only give you the minimum energy required for the LED to operate, but not the actual energy of the LED. Therefore, you should use the voltage that the circuit eventually tends to in order to get the most accurate value of E. As for the second question, you can measure the energy of a UV LED by measuring the energy emitted by it when it is placed in a photovoltaic cell. The amount of energy measured will correspond to the energy of the UV LED.

## 1. What is Planck's constant?

Planck's constant, denoted as h, is a fundamental constant in quantum mechanics that relates the energy of a photon to its frequency. It is a universal constant that has a value of approximately 6.626 x 10^-34 joule seconds.

## 2. How can LEDs be used to find Planck's constant?

LEDs, or light-emitting diodes, emit light at specific frequencies that are determined by the energy bandgap of the semiconductor material. By measuring the voltage and current of an LED, the frequency of the emitted light can be calculated. This frequency can then be used in the equation E=hf, where E is the energy of the photon, h is Planck's constant, and f is the frequency, to solve for h.

## 3. What is the significance of finding Planck's constant?

Finding Planck's constant is essential in understanding the behavior of light and matter at the quantum level. It is also crucial in various fields of science, including physics, chemistry, and engineering, as it helps in the development of new technologies, such as LEDs and solar cells.

## 4. What is the experimental setup for finding Planck's constant with LEDs?

The experimental setup typically involves a voltage source, an ammeter, a voltmeter, and an LED connected in series. The voltage and current of the LED are measured at different applied voltages, and a graph is plotted between the voltage and frequency. The slope of this graph is equal to h/2*e, where e is the charge of an electron. By multiplying the slope by 2 and the charge of an electron, the value of Planck's constant can be determined.

## 5. Are there any limitations to using LEDs for finding Planck's constant?

Yes, there are limitations to this method. LEDs have a relatively narrow range of frequencies, so the accuracy of the measurement is limited. Additionally, the voltage and current measurements may be affected by factors such as thermal effects and fluctuations in the power supply, leading to errors in the calculation of Planck's constant.

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