What I find confusing therefore, is that in certain circumstances, light can be seen to act as a wave. This indicates to my simple way of thinking that a photon of light travelling in direction x spatially, will during that journey alternate between travelling in a positive and then a negative and then a positive y direction, and so on.
If its y direction during its journey alternates between positive and negative, does this not mean that the photon of light is undergoing a number of changes in state during its journey ?
If I understand it correctly, the wave function of light is a probability function (the chance of finding a photon within the area defined by the wave). Once a photon interacts with something then this wave function collapses to just one probability.
In Young's double slit experiment the photon actually "passes through" two slits at once and interferes with itself producing the interference patterns on the detector. I think it is incorrect to picture the photon physically being in the crest or trough at any one time. The photon is actually at the crest and the trough at the same time.