During take offs the flight attendant makes sure that mobile phones are switched off (or maybe turned in airplane mode), why is that necessary? (I suspect that there is something to do with physics that's why it was posted here.)
As others have pointed out, one reason is that the cell towers couldn't really deal with the signals.
The other, and this is more theoretical in nature, is that the working frequencies of the transmitters in the phone, and the CPU and other chips, might give off signals that interfere with the navigational receivers. This isn't entirely crazy, we had a deskside-PDP-11 machine that you could tell was running from the next room because you could hear it on the radio.
That said, the frequencies in question are very widely separated for exactly that reason, and it seems there are no proven examples of this ever happened in spite of millions of hours of phones being left on by mistake.