Behind the exit slit of the monochromator a photomultiplier tube waits to measure the amount of radiation having the selected wavelength. I didn't like when I was inside a photomultiplier, but my electrons were very happy there. The photomultiplier consists of a photocatode, an anode and some dynodes. Each dynode is connected to the next one by a resistor, so that when a dc voltage is applied between anode and cathode a dynode is at a more positive potential than the previous one.
I was on the first dynode in a photomultiplier tube and I saw what happens there. First the light strikes the photocathode. Some electrons (called primary electrons) are emitted and they get some energy from the electric field between the cathode and the first dynode. If you don't take care, the primary electrons hit you and you lose some of your electrons. Anyway, the removed electrons like to be attracted by the following dynode, to hit it, and to remove some other electrons. Being on the first dynode is not a big problem but when you are on one of the last dynodes it is very difficult to avoid incoming electrons because their number is increasing with each hit dynode. This way the optical signal is transformed into an electrical one and amplified to a measurable value.
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