How do neon lights work?

In this activity a model shows how gas discharge lamps work.

Neon and other gas discharge lights are different from incandescent lights. There is no glowing filament. They are cool to the touch. They are inside a glass tube. They produce light more efficiently than incandescent lights. And they come in different colors. Besides neon, there are mercury and sodium types, often used for streetlights.

Describe how you think gas discharge lights work, and why they come in different colors.

1. Here is a model of a gas discharge light. Initially it shows hydrogen, but you can choose other gases. The gas is in a vacuum chamber with electrodes at each end. When you play the model, one electrode heats up and releases electrons. They are attracted to the other electrode by the electric field, whose strength is determined by the voltage shown at the top.
   
2. Each gas atom is surrounded by a cloud of electrons in different energy levels, represented by the lines on the right. The electrons are normally in the “ground state”, which is the lowest they can be in.
3. When an electron hits the atom, an electron can be excited into a higher energy level. When that electron drops back to a lower state, the atom emits a photon of light, represented by a ball with a tail. The energy of the photon is equal to the change in energy of the electron when it drops back to a lower level.
4. Photons with different energies are different colors. Choose the spectrometer option (lower right) to show a spectrum of the photons that are emitted. Beyond the visible spectrum are ultraviolet light and infrared light. Which end is more energetic, and which is less?

1. Open the Neon Lights model.
2. Choose continuous electron production and run the model. Even though hydrogen has five energy levels, there is only one spectral line. Why is that? Is it visible?
3. Increase the electron production to 100%. Are there more spectral lines? Explain.
4. Increase the voltage. This will increase the electric field and the energy of the electrons that are bombarding the gas. What happens?
5. Run the model for a while. How many visible spectral lines does hydrogen have? List their colors and their wavelengths (nanometers).
6. Reset the spectrometer and try other atom types. For each type, note the number of visible spectral lines and their colors.

1. Open the Neon Lights model.
2. Choose multiple atoms at the top of the model. Choose continuous electron production. Run the model with different gases. Note the number of visible spectral lines and their color.
3. There is a configurable option for atom type. See if you can make an atom with no visible spectrum. See if you can make one whose main color is blue, green, or red.

1. Using this model, can you explain why sodium discharge lamps appear orange-yellow?
2. Why are these lamps more efficient than incandescent lights?

Fluoresescent lights that are common for general lighting are gas discharge lamps just like this model. However, they have an added feature: a coating on the inside of the glass tube that fluoresces (glows) when it is hit by the photons that are being emitted by the gas inside the tube. So you don’t see light directly from the gas. Instead you see light emitted by the excited electrons in the coating.

Investigate the differences between gas discharge lights – such as neon – and fluorescent lights. What about efficiency? What about color?