If only one slit is observed in the Double Slit experiment, will the unobserved slit produce an interference pattern? I am having a difficult time solving this. Say that electrons are emitted from a source S at a very slow rate. If both slits S1 and S2 are observed, we would have roughly 50% probability of detecting an electron at one of the two slits. The interference pattern is lost and the intensity distribution will appear as the sum of two individual sources: I = I1 + I2. 
But what if only one slit (S1) is observed? The observed slit (S1) will appear to produce a normal distribution, but what about the unobserved slit? This experiment has been performed with individual electrons, so we know that if both S1 and S2 are unobserved the intensity distribution contains an oscillating term for each electron. Does concluding that an electron must have passed through the unobserved slit count as an observation, and therefore destroy the interference pattern?
Edit: changed the source to electrons
 A: An interesting question, but by its own nature, the true answer is unknowable. If we managed to make some predictions about what happens to the unobserved slit, we would need to still experimentally verify them. But how could we? Any experiment that involves observing one slit and having zero knowledge about what happens at the other would necessarily require us to never observe the slit of interest. So we could never gain the data to verify our predictions. Unfortunately, when quantum mechanics says something is unknown, it usually means we can never know it or even guess at it.
To answer your last question, if you track when an electron is detected and the slit you are observing had none pass by, then deducing that it must have come through the other slit does count as observing it.
A: The electrons of the unobserved slit produce fringes on the screen. Once more, electrons near an edge produce fringes too. That happens because there is an interaction between the edges of the slit plate (correct the material of the edges). Young saw fringes, thought about waves and later we got geometrical equations, satisfying what we see on the screen.
