# B Regarding Young's two slit experiment

1. Aug 9, 2016

### mshawkeyems

regarding Young's two slit experiment
1. HOW can a light source is used to create a single photon that can be directed at 2 slits simultaneously ?
I am confused as to how the light source can be aligned with both of the slits simultaneously.

IF the light beam is wider than the distance between slits can a shutter used to isolate a single photon also be wider than that distance .

2. Aug 9, 2016

### BvU

Hello hawkeye,

Light has characteristics of waves. The better you align a light source with narrow slits, the more the wave front diverges. Think of waves in water: a narrow wave front spreads out, a wide wave front moves almost parallel.

You can't use a shutter or an opening. Google Huygens principle

3. Aug 9, 2016

### sophiecentaur

There are 'single photon sources' available. Have a look here.
Now, you cannot 'fire a photon' in a particular direction. What you can do is to make an optical system (waves not photons) which will produce a beam that covers the two slits in the same way that you would if you were doing the normal Two Slits experiment. A single slot is one way to achieve this, with the two slots at some distance between it and a screen. When you turn down the brightness until individual photons are produced and you look on the screen, you will very slowly build up a pattern of where single photons arrive on it. The pattern turns out to be the same as the pattern with a normal beam. So the place where the photons arrive (one at a time) is still affected by the presence of the two slits. We can 'say' that each photon has interfered with itself. But we don't have to put it that way (imo)
Personally, I also find it convenient to say what's happening is no different for individual photons and that there is a common or garden old EM wave being generated between source and detector. The power of the beam is very low (one photon's worth of energy every second is still a Power value). When the beam hits the two slits, it produces an interference pattern but, as the energy can only be detected in one place (a single photon's worth of energy is all that's available) then where it actually turns up (is observed) is just determined by statistics. This view of things avoids the problem of a photon having to interfere with itself because the interference is still happening with a wave - despite the very low power. I always find it easier to ignore your actual photons until they actually interact with an object (at either end) and then they can be regarded as the 'packet of energy' that Einstein first described in his photoelectric effect studies. But that's little more than an alternative philosophy, which I use because it suits me - and so is a lot of what QM says.

4. Aug 9, 2016

### sophiecentaur

You are talking about the actual optics here. A single slit (what you refer to as a shutter?) will cause a diffraction pattern that is wider and wider as the slit gets narrower you can choose a width which will be narrow enough to include the two slits but not so narrow that there isn't enough light for you to observe the pattern. Of course, the pattern from the two slits is also a diffraction pattern, consisting of the broad pattern of each slit and the fringe pattern of the pair. It's all diffraction when you get down to it.