# Double slit questions from an observation perspective

1. Apr 2, 2013

### GOD__AM

Some questions have been haunting me about this experiment for some time. I'm mainly interested in what we observe from the perspective of the projection screen.

First lets imagine that there is only one slit, and we are observing from the projection screen looking back at the slit with the laser behind it. We would have a direct view of the light source through the slit. It may be more effective to imagine "us" as a tiny camera to help with the scale of the small projection.

Now observing from the same spot we change to the double slit. Now we don't have a direct view of the source light, or maybe we have a view of the left and right side of the source with the middle of the two slits blocking part of our view in which case we still see the source in part.

Now if we move to the right or left into one of the dark areas on the projection screen I assume we don't see the source at all, and we don't see any light coming from the slits. Is this correct?

Next we move farther to the right or left into then next lit area of the projection screen. What do we see now? We should see light coming from somewhere, presumably from the slits, but can we see the source?

What if we move to the right or left extreme to the last light band on the projection screen, where do we see the light coming from and do we see the source of the light?

It seems to me that observing from any lighted area on the projection screen should give us a clear view of the source, however unlikely that sounds, unless the light is reflecting off the side of the slits. Even then we should see the source in the reflection I would think.

Sorry, I wasn't sure which section this question belongs in.

2. Apr 2, 2013

### Staff: Mentor

Effectively the slit is the source. It is diffraction limited, so you won't be able to see anything beyond.

3. Apr 2, 2013

### GOD__AM

Okay, so then moving to either side through light and dark areas looking back at the slit we see it as lit or not depending on our position in the projection screen?

Can we see the space between the slits or does it just appear as one slit?

4. Apr 2, 2013

### Staff: Mentor

That is a good question. I don't know.

5. Apr 3, 2013

### sophiecentaur

This is the nub of the question. If the two slits are narrow enough then you will see light through both of them because you will be in the broad diffraction pattern of each of them (they can be regarded, effectively, as point sources). There is nothing more 'special' or "direct" about light from one than from the other and the amplitudes will be more or less the same. Assuming you are within the broad maximum of both slots then you will see the fine structured fringes with deep nulls (as you move your head from side to side). The simplest analysis assumes you are far enough away for the two 'beams' to be parallel (Fraunhoffer - far field) so the diffraction minima for each slit will coincide. In the near field, they will not coincide so the two-slit pattern will not have very deep nulls in it because the two cancelling phasors will not be of equal magnitude. But this is just a nicety and you should 'get' the far field situation first, if you want to grasp just what is going on here.

OH yes - and whether you 'see' the two slots as separate or not, if you stand where the screen is, will depend upon the resolution of your eye - you may see two slots, one (blurred) slot or even nothing (using just one eye), if you are standing in a null direction.
There is a further question, which would be sure to arise from this conversation and that is - what would you see, if the light level were so low that you could identify just one photon at a time? Would you see one slot or both slots when a photon goes through?

Last edited: Apr 3, 2013