Explain Young's double slit experiment

Answer 1

If coherant light is passed through two slits of the right size and separation then light and dark bands are seen on a screen at the correct distance fron the slits. This demonstrates the wave nature of light. The light is amplified where two crests arrive at the same time and no light arrives when a crest and a throw arrive at the same time. This experiment can also be done with electrons, with similar results.It was later proved that photons are always particles the crests and throws being probibility amplitudes of all the possible paths that a photon can take.

Einstein famously said that if he could ride on a beam of light then the passage of time would appear to stop for stationary observers. As far as the observers were concerned, Einstein would appear to be in several places at the same time i.e. appearing simultaneously at every point along his path of travel. So what does this say about the electrons of an atom which after all whizz round the atom's nucleus at the speed of light? Well, to the rest of the world (i.e. the atom's nucleus, the molecules of which the atoms are part and everything else on the stationary Earth) a single electron of an atom will be simultaneously located at every point around the atom's nucleus, and at a fixed distance from that nucleus. Each of the atom's electrons will thus be the surface of a sphere centred on the atom's nucleus and of a radius equal to the electron's distance from the nucleus.

Now let's consider what happens when the atom happens to be one of those on the surface of a heated light bulb filament. At intervals of time the energy produced by the heat will cause one of the atom's electrons to release a light photon and this will fly off in a straight line away from the atom. But as we have agreed above, an atom's single electron is actually millions of instances of itself, all travelling in different circular directions around the atom's nucleus. So as the electron leaves the atom, all these instances will fly off in every conceivable straight line direction. Such activity is very similar to a wave front i.e. an ever-widening sphere of energy travelling away from the atom; so similar in fact that wave mathematics can be used to successfully analyse it (n.b. the release of each set of photon instances is the quantum of energy that gives its name to quantum physics).

Wave mathematics is still used today to explain the behaviour of light passing through the two slits in Young's famous experiment of the early 1800s. But the real explanation can be found by considering that being of similar charge (negative) light photons will always try to repel one another. In travelling away from an atom however, they will be prevented from doing so by the fact that each photon will have other photons by the side of it, and all around it. The repellent force from each of these neighbouring photons will be cancelled out by an equal force from the photon on the opposite side.

When the stream of photons passes through the slit, however, photons will be held back, leaving just a narrow stream to travel on. The photons in the outer parts of the narrow stream will now be free to move outwards and in fact will be pushed outwards by the repellent forces from photons in the inner parts of the stream. The stream will thus fan out into a new spherical wave-like front. And just like waves, the photons will interact with those opening out from the nearby second slit, resulting in the familiar wave-like interference pattern of regular light and dark bands.