# The double slit experiment

1. Sep 12, 2011

### Misr

in the double slit experiment , why must the two slits be at the same distance from the first slit?should the interfering waves have the same phase and why?

2. Sep 12, 2011

### Staff: Mentor

They don't have to be. It simply makes the analysis a little bit easier.

3. Sep 12, 2011

### Drakkith

Staff Emeritus
I think one of the points of the double slit experiment is to show that there doesn't need to be 2 different waves/particles to interfere with each other, but that one will interfere with itself as it passes through the double slit.

4. Sep 12, 2011

### sophiecentaur

To do the experiment using non-laser light you need to get over the poor coherence of the light. Making the paths as symmetrical as possible will give the best pattern (one with the best defined peaks and nulls).

5. Sep 14, 2011

### Misr

but coherence doesn't mean they have the same phase
According to the textbook "The two slits are at the same distance from the first slit in order to lie on the same wavefront (to have the same phase)"
but I don't understand,if they are at the same distance from the first slit,why should they lie on the same wavefront?

6. Sep 14, 2011

### sophiecentaur

Coherence means that they have the same phase. Photons are produced at random and the light consists of very short bursts. These bursts need to get through the two holes at nearly the same time for interference to occur. A symmetrical arrangement gives the best 'overlap' and the pattern is best. One burst won't interfere with a bunch of other bursts. It has to interfere with itself. (Haha)

7. Sep 14, 2011

### Misr

I can't imagine that
I read before that Coherence means that the phase difference is constant over time

8. Sep 14, 2011

### Born2bwire

Yes, I would disagree with sophiecentaur here. Coherence relates, in some ways, to the idea that the relative phase does not change over time and/or space as opposed to being no phase difference. Still, the way coherence is defined allows for more freedom than that. Basically, coherence describes how correlated two waves are. When one wave changes, the other wave changes accordingly.

For the purposes of the double slit, we mean that the two wavefronts are preferably in phase so that the interference patterns are easily analyzed. In addition, coherence of the two wavefronts also means that the interference patterns are regular and consistent.

To do this, we can have a single point-like light source. If the slits are narrow and are equidistant from the light source, then the slits themselves will act like new point sources (being very narrow their diffraction patterns are like a point or line source). The phase of the sources at each slit would be dependent upon the path length from the original light source. So if the paths are the same, then each slit will behave like two sources that are in phase.

Take for example the following case that I have on youtube:

The reason why we do not see a strong interference pattern is that because of the difference in paths the amplitude of the wavefronts coming from the two slits differ so the interference is weakened. But we can see a regular interference pattern arise on the left hand side.

9. Sep 14, 2011

### sophiecentaur

Coherence is not an absolute. A laser or a radio.transmitter are very coherent sources. Phases measured at very different places will not change. An ordinary light source, as I said, is low coherence, consisting of many individual unsynchronised sources. Without lasers, you can't make holograms, for that reason.
Imagine a number of radio transmitters in a small space, operating at nearly the same frequency. If you put a Large screen with two slots (large), like a scaled up young's slits, you would measure maxes and mins in appropriate directions. But the pattern would not be as good as for one single source with fixed frequency.

10. Sep 19, 2011

### Misr

I think that Both of you are right because waves in phase are called "coherent" because the phase difference -which is zero-is constant over time.

11. Sep 19, 2011

### sophiecentaur

There is little point in using a simulation to 'prove' anything unless the maths of the simulation is spelled out.

There are two reasons for a perfect interference pattern being corrupted.
1. Amplitudes not being equal.
2. Lack of coherence.

For two adjacent slits, the amplitude difference wii be negligible due to the distance (being more affected by relative slit widths)

It just has to be due to coherence length (look it up).
Two Slit interference will work fine using laser light whatever direction it arrives from. It does not work so well for off axis arrival when the light has a poor coherence length - or imprecise / mixed wavelength i.e from a non laser source. What happens here is that the phase of the light entering one slit varies in time relative to the phase of light through the other slit. By situating the source symmetrically, you ensure that the phase relationship is much nearer constant which gives a better pattern.