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With the advancement of technology since the 19th century, we can now perform more advanced versions of the dual slit experiment. These modifications involve isolating the particle of light known as a photon and sending that through the slits. Furthermore, there is more versions where it is attempted to measure which slit the photon passes through. Both of these experiments reveal strange things about the nature of light.
The existence of light that existed in countable particles was proven by Einstein in 1905 [1] using the work of Planck on black body radiation. After the existence of these units of light were observed experimentally there was a revival in the particle nature of light, because waves are continuous, and don't have quantized units.
This new property of light required that the dual-slit experiment be revisited, but this time with quantized light. The big question to answer was "do light particles behave like waves in groups and like particles when alone?". In order to test this question, the dual slit experiment was modified by reducing the intensity of light until only one particle was released at a time. With only one particle at a time, there was no way for the light particles to interfere with each other. So, if we were using a classical explanation, it would follow that there would be no interference pattern.
The actual experiment was performed in 1909 by Taylor, where he reduced the intensity of light until the particles were not overlapping. Because the particles are coming individually, the idea of intensity of light doesn't work. Instead Taylor measured the incidence rate of a particle hitting a given spot over a period of time. Using this Taylor can get the probability of a particle hitting a given spot. Since intensity of light is the number of photons that hit a given spot, probability can be used as an analog to intensity, since a spot with a higher probability will receive more light.
The results of the experiment were confusing. Despite light being quantized, and the light being released in a manner that would prevent individual particles from interfering with each other, the interference pattern emerged again. Below is a picture of the build up of photons.
The results of this experiment are baffling if looked at from a classical perspective. Logically, one would think that the photon must go through one slit or another, and therefore the pattern should just be two bands. Clearly, this isn't the case, and so one must think about the problem in another way. Most of these interpretations will be discussed in the [Dual Slit Mark II] (mark2.html) page.
