(Often, physicists prefer to be cool. Most of the time, we use the Greek letter ν (it’s not v) for frequency. It seems more sophisticated to write it that way.)
With this connection between wavelength and frequency, we get this varied energy equation:
It turns out that it is easier to think of the interaction between light and matter than of frequency rather than wavelength.
Well, it was a setup for an experimental method to determine the value of Planck’s constant. The basic idea here is to use the color of an illuminated LED to demonstrate this energy-wavelength relationship. If I can find the amount of energy required to produce light, as well as the wavelength of light produced (in other words, color), I can determine H.
There are some small strategies involved – so let’s get to that.
Power and LEDs
LEDs are everywhere. Both the flashlight on your smartphone and the new light bulb in your room are LED. The red light in front of your television – it’s an LED. Even your remote uses an LED (although it is an infrared). LEDs come in a variety of colors. You can easily find red, yellow, green, blue, purple and more.
An LED is a semiconductor device that has an energy gap, often called a band gap. When the LED is connected to a circuit, it starts the flow of electrons. The energy difference is just like the conversion of that energy into hydrogen atoms. There may be electrons on either side of the band gap, but not in the middle. If an electron has the right energy, it can jump across the band gap. And since the electron loses energy in making the jump, it makes light. The wavelength, or color, of this light depends on the size of that band gap.
If you connect an LED to a single D battery with a voltage of 1.5 volts, nothing happens. To make the LED flash, you need to increase the voltage to a certain value – this is called forward. Red LEDs typically require 1.8 volts and blue LEDs take about 3.2 volts.
Let’s measure this value. Here is my experimental setup. I have a variable power supply with an LED. I can gradually increase the voltage and measure the electric current. Only when the current starts to increase will you be able to see the visible light.