In a light source (like filament of bulb) there are various atoms and as such the things are deemed to be randomised. So there must be a phase difference between each of the wavetrains emitted by the atoms. Logically is it correct to argue that there would be a wave having same amplitude but phase difference of pi corresponding to each wave? Obviously this must be wrong otherwise by supresposition of all these individual waves, net effect would be zero, and we wouldn't be able to see!
P.S. For the very valuable readers, I wish to request that it is a high school students struggle of understanding things, and if my question is a bit too stupid, do please say it.