# Double slit experiment - what defines the distance between slits

Bit of a physics newbie but I can't find the answer to this anywhere.

When conducting the double slit experiment it seems that the distance between the slits is somewhat important and my reading suggests 0.15mm. Certainly I made one using a laser pointer and two manually cut slits and it didn't produce results to start with until I narrowed the gap sufficiently.

But the question really is - what is the maximum distance where a photon wave will effectively go through both and interfere with itself which then leads to "how wide is a photon?" which I understand to be a meaningless question.

There is no upper limit. The fringe spacing is given, approximately, by

$$\frac{D\lambda}{d}$$

where $D$ is the distance from the slits to the detector, $d$ is the separation of the slits and $\lambda$ is the wavelength of the light.

So, how large $d$ can be depends on how long you can make the interferometer (how big $D$ is) and what the wavelength of the light you are interested in is. If you're interested in 'light' which is microwaves or longer wavelengths, then $d$ can get pretty big.

• So are you saying that the photon is effectively travelling in all directions? If I have 2 slits 100 miles apart the photon will still go through both? If I put the slits behind the laser then the photons will still go through both? – Gwynge Jan 17 '18 at 16:25
• There is no limit to how 'large' a photon can be, no. – tfb Jan 17 '18 at 16:30
• Gwynge, welcome to quantum mechanics. It's weird, isn't it? But to say something constructive.....the reason for this has to do with the idea of probability waves. You can think of it a bit like this....You drop a rock into a pond and the waves go out in all directions. Same thing happens if you disturb the EM field. A photon "goes out" in all directions. – Ben Jan 17 '18 at 17:43

In answer to a comment above

So are you saying that the photon is effectively travelling in all directions? If I have 2 slits 100 miles apart the photon will still go through both? If I put the slits behind the laser then the photons will still go through both?

Photons are elementary particles and in the standard model of particle physics they are point particles. They are not all over the place, classical light emerges from a superposition of the wavefunctions of a large number of photons.

Here is a double slit single photon at a time experiment

Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.

On the left each dot is the footprint of an individual photon, and it is not spread out all over the place. At the right is the probability distribution which coincides with the classical interference pattern, for the quantum mechanical problem "single photon impinging on double slits".

The photon is not "traveling in all directions", it is being scattered by the fields of the sides of the slit and there is a probability for an individual photon to scatter at a large angle. The probability depends on the photon energy, $h.ν$, and so the distances are related to the wavelength that will be built up by many photons, if one wants to see a clear interference pattern.