Originally Posted by

**RGSP**
No, either you or your physics teacher has confused 90 degrees in phase with 90 degrees in position. The easiest and almost universal way of characterising AC waveforms is by their phase, where the instantaneous voltage (say) will be given by:

V(inst) = V(peak) X cosine(theta)

where theta is the phase angle. If you don't like sines and cosines, no worry, the key thing is that as this characteristic angle goes from 0 to 360 degrees it does through two peaks (when light will be at max), one positive at 90 degrees, and one negative at 270 degrees, and it will go through zero twice, once at 0 and once at 180 degrees, when the light output will go to zero.

By using capacitors and inductors you can shift the phase of an ac power source, and it was common to operate fluorescents in pairs: one run from the unshifted mains, and one from a 90 degree shifted source, so that the peaks of light from one tube coincide with the dark points of the other, and vice versa.

You have to order in such pairs specially, and to be honest I'm not sure whether you can still easily get them, because high frequency fluorescent fittings have mostly taken over, and have got to be only moderately expensive.