How come we dont see light interference from two light sources?
ex. two candles
There has to be a definite phase relationship between light from a source following different paths to produce an interference pattern. There is no definite phase relationship between light from two sources.
Light emitted from regular light sources such as light globes, candles, etc... undergo very rapid phase changes, about 10 every billionth of a second. So although the light from two candles do actually interfere with each other (if they didnt then the emitted light would not obey the laws of physics now would they!?), the periods of destructive interference last for about 10 billionths of second. The result is that no interference effects are observed by the human eye and the light source is termed incoherent.
So we do not see light interference from two candles because our eyes are not sensitive enough to see it (and this is a result from the fact that there is not a definite phase relationship - as Older Dan has said).