Why does a battery operated transistor make noise when a nearby tube light is switched on?

A conventional fluorescent lamp while switching on/off generates electromagnetic waves with frequency around 1400 kilo-hertz lying in the medium wave frequency region. Hence this can be considered as a mini radio-frequency generator.

Transistor radios (radio receiver to be more precise) operating especially in the medium frequency can pick-up this radio-frequency interference in the same fashion as they do while switching an ordinary electrical switch on/off. In particular this phenomenon is more pronounced in fluorescent lamps employing glow-switch starters operated to initiate the electrical discharge process inside the column of the fluorescent lamp.

This interference has profound dependence on the distance between the fluorescent lamp and the radio receiver, which varies as inverse square of the distance between them. This interference can also propagate via electrical connections leading to main cables so that this can travel longer distances.

Radio receivers can be made to overcome this interference by proper grounding or constructing an antenna of suitable design and length. That is why a radio operating in a properly grounded main electrical system does not pick-up this interference.

Another way to overcome this interference is to connect a small capacitor across the lamp terminals along with stabilisation of self-inductance (from the choke coil of the ballast). Hence in order to overcome this interference from radio waves it can be seen that a small capacitor is connected across the terminals of the glow-switch starter and the combination is put inside a canister for being used in fluorescent lamp fitting.