What happens when one of the double slits are closed? I was reading "50 quantum physics ideas you really need to know" (by Joanne Baker) and came across an ambiguous paragraph. The topic: Wave-particle duality. It describes a photon that passes through the double slits; that if one of the slits was closed after the photon had passed through the slits but before it hit the detector, "it will behave in the correct way". Well, what way is it? Does the photon interfere, or doesn't it? I guess that it interferes, but I'm not sure.
Experimental results should suffice; being a layman, I might not understand the theory.
 A: She means to say that it would behave as if both the slits were open and fall on the screen according to the double slit pattern. The entire double slit pattern comes from interference.
A: First at all light is electromagnetic radiation. To produce EM radiation you need excited electrons (or protons or some of this stuff), they emit photons. The light you see is thermal radiation from sun, laser, LED, light bulb. Then ever one detect light in detail he will end with the detection of photons. The first who claimed that light consists of quanta - later called photons - was Einstein. More he claimed that photons are indivisible particles from the emission until the absorption.
Amongst other things they are countable. The detection of photons one call direct evidence.
To come to a satisfying answer with the double slit we start with an edge. It is well known that in an area left and right from the geometric position of the shadow line there are fringes, more intensive near the geometrical position of the shadow line and extinguishing to the left and to the right. This one will see if he use a point like source and - then less the source point like is - a sufficient distance of the source from the edge. If one use a tiny source (or a collimated beam) and once more followed the advice to set it far away from the edge how one is bewildered to see fringes in some area left and right the geometric line of the shadow. This effect primarily has to be called diffraction. The fringes are intensity distributions. Repeating this experiment with electrons the result are fringes to. The difference is that the fringes do not appear in the direction of the shadow, "behind" the edge. That is because the electrons from the surface of the edge and the flying electrons repel each other when the photons have an oscillating electric (and magnetic) field and half the time are attracted and half the time are repelled from the surface electrons.
Being a child of modern science one knows that looking deep enouth all around us is quantized. Not atoms and subatomic particles only but fields too. It is without doubt that the edge and the photons are interacting and the interaction takes place through a field. Fields are quantized and the fringes are the manifestation of the structure of this field interaction.
Last step we have to do is to approach two edges close together and we could see a symmetric pattern of fringes. Now approach two slits (made from 4 edges :-)) close together and one see waht? - fringes from 4 edges.
Young can't see this. What he saw was a weak analogy to a moving interference pattern from two sources (or one source and two slits) in water. Only if one animates his drawings it gives the full picture! Fringes are not moving. They are made from photons which are distracted in a quantized way from their almost straight movement through space.
