In photos of a cathode ray tube, the path the electrons travel across the tube glows as a thin green line. A cathode ray is a stream of electrons, and electrons are invisible, so what is that line? Quick answer: the line is not the electrons. It is the gas the electrons pass through.
Why is the line visible
As an electron flies along its path, it collides with the residual gas molecules around it and excites them. When an excited gas atom drops back to its ground state, it emits light. So the gas lights up along the track the electron took, and to the eye that reads as a green line drawing the beam's path. The electron itself never becomes visible.
One more point. An electron moving straight at constant speed does not emit light on its own. Radiation requires acceleration, and a straight, steady electron has none. So all the light in that line comes from the gas or a phosphor.
Why green specifically
The color of the line is set entirely by the gas. A green path means the tube contains helium. The fine beam tube (the e/m measurement tube) that bends the beam into a circle with Helmholtz coils is filled with low-pressure helium on purpose, precisely to make the beam show up as a clean green line.
In a perfect vacuum, the line disappears
For the path to glow, there has to be gas to excite. Pump the tube down to a true vacuum and the path shows nothing. This is also the heart of the cathode ray's history.
Observing cathode rays was a fight over how much air you could remove. In 1838 Faraday put a high voltage across a partially evacuated tube and saw a discharge glow. In 1857 Geissler pumped down to about \( 10^{-3} \) atm, but enough gas remained that electrons collided every few millimeters and never formed a fast beam. In the 1870s Crookes emptied the tube below \( 10^{-6} \) atm, the mean free path grew long, and electrons traveled fast in a straight line without collisions: the cathode ray. In 1904 Fleming's thermionic vacuum tube replaced this unreliable cold-cathode method.
As the pressure drops, the green line along the path actually disappears. Instead the high-energy electrons travel straight and strike the end glass, and that glass fluoresces as a green spot. A spot, not a line, and its shade is set by the glass composition (soda-lime and lead glass fluoresce yellow-green). So: with gas present, the path shows a green line; near a perfect vacuum, the end glass shows a green spot.
Go the other way, to higher pressure (the Geissler regime), and the gas is so dense that electrons collide constantly and never form a clean beam. Now you get a glow discharge, with bright regions and dark spaces alternating, and again the color is set by the gas (air gives pink or purple).
The fluorescent screen
There is a second way to produce a green line, and it works differently from gas emission. A Teltron deflection tube is at high vacuum, so there is no gas along the path to light up. Instead a mica plate is set at about 15° to the beam axis, with a phosphor screen on one face and a centimeter scale printed on the other. When the beam grazes this plate, the spot where it lands fluoresces and draws a straight green line on the plate. That line sits on the plate's surface, not in open space.
The two methods look the same but are not. Gas emission lights the trajectory in three dimensions in open space; the fluorescent screen draws a flat line on a surface. For reference, the green-emitting phosphor is ZnS:Cu (copper), while silver-doped ZnS:Ag is blue.
Recap
Either way, what you see is not the electron. It is the light from what the electron touched, either the gas or a phosphor. The electron stays invisible from start to finish. This principle collected into one device is the CRT. A very-low-pressure tube fires an electron beam at a phosphor to paint the screen.
References
- Harvard University, Natural Sciences Lecture Demonstrations (n.d.). Maltese Cross CRT.
- University of California, Santa Barbara, Physics Lecture Demonstrations (n.d.). Cathode Rays (88.06); Glow Discharge (88.03).
- Institute of Physics, IOPSpark (n.d.). Electron Beams (Cathode Rays); Types of Electron Tube.
- 3B Scientific (n.d.). Electron Deflection Tube D (Product Specification).
- Wikipedia (n.d.). Teltron Tube.
- Physics Forums (2012). Cathode Ray Tube: Explaining Phenomena with Green Beam and Fluorescent Glow.
- U.S. Philips Corporation (n.d.). Green-luminescing Zinc Sulphide and Cathode Ray Tube Provided with This Zinc Sulphide (US Patent 5,558,817).