# Why aren't interference patterns wiped out by random phase shifts?

In the double slit experiment with light, we assume that the two waves have exactly the same phase when exiting the slits - but this seems unrealistic. Why don't random phase shifts at the slits wipe out the interference pattern?

• What do you think would cause such a random phase shift? It's true that the double slit experiment is hard, and you need coherent light for it to work well, but I see no reason the slits themselves would be a problem. From the light's perspective, the air inside a slit is just the same as air everywhere else. – knzhou Jan 24 '17 at 19:17
• The interference is washed out by random phase shifts! This is why the experiment is done with a laser, a.k.a. light without random phase shifts. There is some nuance here: light is not either completely incoherent or completely coherent, and you can see interference with light that is not perfectly coherent. A good answer to this question might show how the properties of the interference fringes depend on the coherence time of the light source etc. – DanielSank Jan 24 '17 at 19:20
• What makes the radiation behind slits coherent? – HolgerFiedler Jan 24 '17 at 21:51

## 1 Answer

The light waves which come out from the slits will have a constant phase difference which could be zero.
The phase difference depends on the angle the incoming light makes with the plane containing the slits.
If the incoming light was at right angles to the plane containing the slits then the light emerging from each of the slits would be in phase.

All a constant phase difference between the light which comes out of each of the slits will do is to shift the position of the interference pattern.

• The two slits are illuminated from behind with a lamp that supplies the light. This lamp is large and light from all parts of the filament will reach the two slits. The different parts of the filament radiate completely out of sync. – yippy_yay Jan 24 '17 at 21:21
• @yippy_yay If you look at the set up for observing twin slit interference using a conventional light source you will find that the source has a single slit in front of it. The light which passes through the single slit illuminates the twin slits. – Farcher Jan 24 '17 at 23:06
• @Farcher The question stays the same, what makes the radiation behind this first slit coherent? – HolgerFiedler Jan 25 '17 at 5:49
• @HolgerFiedler The first slit is an attempt to make the source look like a point. If the slit has a finite width the double slits produce displaced fringe patterns depending on where the light emanating from the single slit has come from. Thus the final fringe pattern produced by the double slits are less distinct. This may help: physics.stackexchange.com/questions/114926/… – Farcher Jan 25 '17 at 8:07
• @Farcher in your link I gave the same answer as you;-) . Some people explain the coherence of the double slit experiment with the first slit and this is nonsense because this shifts the question about coherence only from the double to the first slit. BTW the interference pattern of water waves along an observer line moves to the right and left. The intensity distribution of light is static. That's strange. – HolgerFiedler Jan 25 '17 at 17:10