Television tubes use a combination of phosphors to produce color. Each phosphor emits a specific color of light when struck by electrons.

For the color red, the phosphor used is typically europium-activated yttrium oxide (Y2O3:Eu).

Here's how it works:

* Electron Beam: The electron gun in the television tube shoots a stream of electrons at the screen.

* Phosphor Coating: The inside of the screen is coated with tiny dots of different phosphors.

* Light Emission: When electrons hit the phosphor dots, they excite the atoms within the phosphor. This excited state causes the atoms to emit light at a specific wavelength, which corresponds to the color of the phosphor.

So, europium-activated yttrium oxide (Y2O3:Eu) is the key ingredient for producing the red color in older television tubes.