You can measure this fundamental quantum constant using LEDs

(Often, physicists like to be quiet. Most of the time, we use the Greek letter ν (not v) for frequency. Writing it this way seems more complicated.)

With this correlation between wavelength and frequency, we get this modified energy equation:

Illustration: Rhett Allen

It turns out that it’s easier to think of the interaction between light and matter in terms of wavelengths rather than frequency.

Well, it was all just setting up an empirical method for determining the value of Planck’s constant. The basic idea here is to use luminous LED colors to demonstrate the wavelength-energy relationship. If I can find the amount of energy required to produce the light, as well as the wavelength (in other words, the color) of the light produced, I can determine NS.

There are a few little tricks involved – so let’s get to it.

Power and LED lights

LEDs everywhere. This light bulb on your smartphone and the new light bulb in your home are both LED lights. The red light on the front of the TV – it’s an LED. Even your remote uses an LED (though It’s infrared). The lamps come in different colors. You can easily find red, yellow, green, blue, violet and more.

LED is a semiconductor device with an energy gap, often called a band gap. When an LED is connected to a circuit, it starts a flow of electrons. The energy gap is similar to the energy transfer in a hydrogen atom. Electrons can be located on either side of the band gap, but not in the middle of it. If an electron has the right energy, it can jump across the band gap. As the electron loses its energy during the jump, it produces light. The wavelength or color of this light depends on the size of the band gap.

If you connect the LED to one D battery with a voltage of 1.5V, nothing will happen. You need to increase the voltage to a certain value so that the LED indicator glows – this is called forward. Red LEDs usually require about 1.8 volts and blue about 3.2 volts.

Let’s actually measure this value. Here is my experimental setup. I have a variable power supply connected to an LED. I can slowly increase the voltage and measure the amperage. When the current begins to increase, then you will be able to see visible light.

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