Four-D-Topology said:
I have also puzzled about this many times. As you say above, "wherever there's destructive interference somewhere there's constructive interference somewhere else", but I've never understood how the energy is 'diverted' into those constructive areas from the destructive ones.
I mean, a light ray (and the energy it contains) travels along a straight line so how is the energy from the destructive areas shoved aside (forwards, backwards, left, right, up or down) to an adjacent constructive area??
If these were solid particles instead of photons (or waves) I could see them 'bouncing' off to the sides, but clearly that is not happening, so what is 'effecting' the transfer of energy away from its straight line path??
It seems to me that the problem here is simply that your basic assumption is wrong. Light (or rather the energy stored inside the light field) simply does not travel along a straight line as a fundamental law. This is the exception rather than the rule, although it is a very common thing to happen.
Huygens' principle might be the kind of visual explanation that helps you.
https://en.wikipedia.org/wiki/Huygens–Fresnel_principle
If you throw a stone into a pond, the water wave will not move along a line, but the wavefront will form an expanding circle. In order to have the wavefront travel along a straight line, you will also need to throw something that resembles a line into the water. On a classical level, light behaves somewhat similar. Huygens' principle (or rather the Huygens-Fresnel principle) describes this in a rather empirical manner. If you place some symmetric light emitter somewhere, it will also not emit lines of light, but will equally emit into the full solid angle. If this emitter is point-like and you have a look at the emitted wave, you will find that it somehow resembles the wave seen then throwing a stone into a pond. You get a spherical wave moving outwards from the emitter, not a straight line. Huygens principle now simply states that you get the resulting light field by assuming radially expanding light waves and adding up all of the waves at one point (considering phase). Every point reached by light is then also the source of another radially expanding wavefront. This way the whole light field can be constructed. You can easily do this yourself using a pair of compasses.
Now how can this be in line with your everyday observation of light traveling in straight lines like (to first approximation) for a flashlamp? If you place a lot of emitters along a line and apply just the construction scheme mentioned above (again, you can do this simply by drawing circles around each point emitter), the lines of the circles represent points, where the light fields are in phase. These are the wavefronts. You will find that this gives you a wavefront that is parallel to the initial line of emitters. The wavefront kind of reproduces itself. All these circular partial waves interfere such that the initial wavefront seems to move forward in a straight line. You will only see deviations from that linear propagation at the edges of the initial line, where you can actually see that the underlying expanding waves are expanding circularly as they move to the side.
Now transport of energy essentially follows the direction of phase gradients. As I told you before, for the line geometry, you will also get lines of constant phase parallel to the initial emitter line. As phase is constant along this line, you will only get phase gradients and energy transport in the direction perpendicular to this phase front line, which corresponds to straight linear propagation of energy. However, this requires the absence of other mutually coherent fields, which might interfere with your propagating light field and will drastically change the phase gradients. So for small regions in space, linear propagation is the exception rather than the rule - however the absence of other fields is a condition that is fulfilled very often.
