Measurement problem in double slit experiment

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In double slit experiment of electron, we see an interference pattern when we don’t know through which slit electron has passed and we don’t see an interference pattern when we know through which slit electron has passed. Now, to identify the slit through which electron has passed, we use photons. We see at slits using photons and this act destroys the interference pattern. We say that interaction of photon with electron is destroying the interference pattern. But photons are always colliding (and hence interacting) with electrons while it is passing through the slits. So, how come this interaction affects the interference pattern only when we catch the photon i.e. only when we observe the electron and not when photon is simply colliding with electron and we don’t catch that photon?
 

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dx
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Doesn't matter if the photons are caught or not. The interference pattern is destroyed as long as there is light present with a wavelength that is small enough to locate the electrons with sufficient accuracy to distibguish between the two slits.
 
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Doesn't matter if the photons are caught or not. The interference pattern is destroyed as long as there is light present with a wavelength that is small enough to locate the electrons with sufficient accuracy to distibguish between the two slits.
thanks a lot for the reply. but I have some doubts. how do you know that this will happen? is there any experimental evidence or any theoretical analysis? if whatever you are saying is correct, then it means we have to do the experiment in dark. in the presence of light, we won't get interferance pattern whether we try to locate the electron or not. (can you give some referances where this is discussed)
 

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