Orange LED in Liquid Nitrogen [Video]

We submerge an orange LED into liquid nitrogen which causes the LED to change colour from orange to yellow and towards green.

So geeks, can anyone here explain exactly why the light emitted by the LED changes color when submerged in liquid nitrogen? If you do, let us know the explanation in the comments section below!

[Matthew Rollings]

13 Responses to Orange LED in Liquid Nitrogen [Video]

  1. I’m only guessing, but I suspect that the super-cooled temperature decreases the resistance of the circuit and correspondingly increases the current through the diode. More current means more power and therefore more energy in the same amount of time. The increased energy has to go somewhere, so the LED emits higher energy photons, pushing it toward the upper end of the visible spectrum. Orange turns to yellow and even green. Just a guess, but it seems reasonable.

    • Actually, Matthew’s explanation is wrong. Firstly, unlike some other circuit components, LEDs have higher resistance at low temperatures, and lower resistance at high temperatures (which is why they burn out so easily if you run too much current through them without a resistor in the circuit – the temperature in the LED rises, the resistance drops, the current increases, the temperature rises more, then yr LED is dead).

      Secondly, the colour of an LED does not depend on the current. It depends on the energy required for a single electron to cross the LED.

  2. From Wikipedia:
    This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor.

    So it would seem to me that the color is changing because the energy gap is getting altered in the semiconductor as it is cooled to extreme subzero temperatures.

    • Matthew is not at all right. If he were changing the current through an LED would change the color – in reality it changes the brightness. Andrew has it right, as the semiconductor temperature is changed the energy needed to move an electron between orbits is also changed. When that electron drops back down to it’s normal orbit, it emits a photon with the energy difference between the two orbits. Energy in the photon is (speed of light * Planck’s constant)/wavelength. Since only whole photons can be emitted and electrons can only move between whole orbits (no fractions allowed) the only thing that can change when the energy between orbits changes is the wavelength which determines the color.

  3. 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.

  4. physics is (as usual) right.
    in the anode electrons are spread over the valence band according to their thermal energy. because with higher thermal energy the electrons can reach higher energy levels within the valence band. so if you cool the LED the electrons in the valence band will no longer high states. if now an electron comes from the conduction band of the cathode it can jump further down into the valence band, thereby emitting light of a higher energy.
    and in case you wonder: i think it is possible for an electron to do that jump in several steps, for example making the jump from room temperature first and then doing the rest in another step.

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