Imagine a scene where you have a copper wall in front of you, two parallel slits in the wall, and behind the wall is a "wall of sand" that will be "absorbed" by anything that hits it. If you shoot a machine gun at a double slit, there is a chance that the bullet will pass through the slit and hit the wall behind you. As the number of bullets fired, the wall will absorb more and more bullets. Suppose the muzzle of the gun is often misaligned and not very accurate, so the bullets you shoot may deviate in various directions, and the bullets may deflect as they pass through the slit, so the bullets behind the wall do not gather in one point, but are distributed in a certain way.
Close one of the slits and shoot at the other to get two sets of bullet distribution data p1 and p2, then open both slits and shoot at both slits at the same time to get the bullet distribution data p. We found p p1 p2.
Let's change the experiment. There is an electric light source at S, an opaque wall to the right of the light source, two slits in the wall at S1 and S2, and a wall behind this wall.
Close the S2 slit so that the light passes through only the S1 slit, and the light intensity distribution I1 on the wall can be obtained. Then, close the S1 slit so that the light passes through only the S2 slit, and the light intensity distribution I2 on the wall can be obtained.
Finally, open the two slits at the same time, let the light pass through the two slits at the same time, and the light intensity distribution i will be obtained on the wall, at this time i i1 i2?
The answer is no!
The light that passes from S1 and S2 both comes from S, and the distance from S to S1 and S2 is equal, so we can consider the light at S1 and S2 to be "identical". For different positions on the receiving wall, that is, when two beams of light intersect at different positions, they each take a different path path.
If the light is fluctuating, the path length is not the same, and the corresponding phase is also different. So when two beams of light are added together, the light intensity is not simply additive, but there is an additional interference term. The magnitude of this interference term is related to the phase difference between the two beams of light, which has both positive and negative values. When a positive value is taken, the combined light intensity is stronger than the direct addition of the light intensities of the two beams;When a negative value is taken, the combined light intensity is 0. Therefore, the light intensity on the wall will change with the change of position, there are light and dark, and there is a certain regularity, that is, light and dark stripes.
Therefore, if the result of the double-slit experiment of light is a fringe between the bright spots, then it proves that the light is fluctuating. Double-slit experiments can verify wave properties, not only for light but also for electrons. The double-slit experiment on electrons is very classic, and interested readers can refer to Feynman's Lectures on Physics, Volume III.