### When the LED is immersed in the liquid nitrogen, the colour changes!

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Teachable Topics:

• light emitting diodes
• electron band theory
• semiconductors

Theory:

To understand the colour change, you first need to know a little something about how an LED works.

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 a conduction band which allows electrons to flow easily. The other side, called the anode side, has a 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 the difference in energy between the two bands, and this difference dictates the colour of the LED. 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 of the emitted light.

When an LED is immersed in liquid nitrogen, the electrons lose thermal energy. 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.

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.

Apparatus:

• Liquid Nitrogen
• Gloves
• Safety Glasses
• LED
• DC power supply
• Stick (to attach the LED to)
• Wires

Procedure:

1. Attach the LED to the stick and hook the ends of the wires up from the LED to the power supply.
2. Turn the power supply on.
3. Slowly dunk the LED into the liquid nitrogen, submerging completely.
4. Wait and watch as the LED shifts from one colour of the spectrum to the next.

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.

Safety:

• Be very careful when using liquid nitrogen. Wear safety goggles. Don't let it touch you or your clothes.