How fluorescence light work

Hi, so long have not update my blog. Maybe I did not encounter any special science expt which I can write on. Finally, today I encounter one. My lab have a fluorescence light which half of it is with the white coating while another half is without the white coating. My previous lab tech have ever shown me that when that lamp is lighted up the part without the coating will not have white light. I did not bother to find out why that time but now I have to teach my new lab tech so I went to check out what is the theory behind it.

The fluorescent lamp is a sealed glass tube. The tube contains a small bit of mercury and an inert gas, typically argon, kept under very low pressure. The tube has two electrodes, one at each end, which are wired to an electrical circuit.

When the lamp is on, the current flows through the electrical circuit to the electrodes. There is a considerable voltage across the electrodes, so electrons will migrate through the gas from one end of the tube to the other. This energy changes some of the mercury in the tube from a liquid to a gas. As electrons and charged atoms move through the tube, some of them will collide with the gaseous mercury atoms. These collisions excite the atoms, bumping electrons up to higher energy levels. When the electrons return to their original energy level, they release light photons in the ultraviolet wavelength range.

As our eye cannot register ultraviolet photons, these light photons have to be converted into visible light with the help of phosphor which is the coating surrounded the tube. Phosphors are substances that give off light when they are exposed to light. When a photon hits a phosphor atom, one of the phosphor's electrons jumps to a higher energy level and the atom heats up. When the electron falls back to its normal level, it releases energy in the form of another photon. This photon has less energy than the original photon, because some energy was lost as heat. In a fluorescent lamp, the emitted light is in the visible spectrum -- the phosphor gives off white light we can see.

This is the reason why the part without the phosphor coating will not light up, as we cannot see UV light. Therefore it is also quite dangerous to look at that lamp as it is different from those UV lamp (black light). Those UV lamp have a black coating which is make up of different type of phoshor coating which absorb the harmful UVB, UVC and visible light but emitting benign long-wave UVA light with some blue and violet light. Whereas my lamp without any coating are emitting all UVA and harmful UVB and UVC.