50. Energy Levels

Students can see the spectral lines of hydrogen by looking at a arc tube on the lecture table through replica gratings. Several students can come down at once and look. You can also project the spectrum of mercury on a screen for the whole class to see (See Projected Mercury and Continuous Spectra).

A simple demonstration of energy levels can be done with 4 LEDs, a handcranked generator, and a supercapacitor. As the capacitor is charged by the generator, first the red LED lights, then the green, then the blue. As the capacitor discharges, first the blue led goes dark, then the green, then the red. There is also a fourth IR LED which can't be seen by the eye, but can be seen on a cell phone camera or video camera when the room lights are out. It turns on first and turns off last. Each color has an energy (voltage) threshold which is related to the photon energy. The IR turns on at 1.2 V, red at 1.5 V, green at 2.0 V, and blue at 3.5 V.

The Frank-Hertz Experiment shows atomic energy levels, but it is a very complicated demonstration.

Finally, a very simple demonstration of energy levels is fluorescent and phosphorescent materials with an ultraviolet light. The energetic UV light kicks the electrons up into high levels, and as they jump part way down immediately (fluorescent) or with some seconds of delay (phosphorescent with partially forbidden transitions), the electrons emit visible light of various colors. A green phosphor requires a blue or ultraviolet light to be activated. A red or green LED or laser won't make it glow, but a blue LED will.