When the LED is immersed in the liquid nitrogen, the colour shifts from orange to yellow to green.
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To understand this colour change, you first need to know a little something about how an LED lamp is constructed. The most important part of the LED is the semiconductor material that the electrons flow through to create light. This semiconductor is manufactured so that is has two sides. One side is called the cathode side. This side has something called the conduction band which allows electrons to flow easily. The other side, called the anode side, has something called the valence band. This band is full of "holes" that electrons fall into as they pass from the conduction band of the cathode side to the valence band of the anode side. These two bands are separated by what is known as a bandgap. The bandgap is simply the difference in energy between the two bands, and this distance dictates what colour the LED will be. This is because as electrons fall from the conduction band to the valence band, they release photons of light energy. Therefore, the size of this drop (the bangap), dictates what the wavelength and frequency is of the emitted light.
When an LED is immersed in liquid nitrogen, the electrons lose a lot of thermal energy, even when the light isn't turned on. When this happens, the bandgap in the semiconductors increases. Since this gap is increased, when electrons in the conduction band fall to the valence band, they emit a higher energy light, meaning the light emitted has a shorter wavelength and a higher frequency. This is why we see the orange light turn into colours that are higher on the electromagnetic spectrum when it is frozen in the liquid nitrogen.
TIP: Doing this experiment more multiple times with the same LED may cause it to break or lose the effect. It's good to have several LEDs on hand.
• Be very careful when using liquid nitrogen, wear safety goggles. Don't let it touch you or your clothes.