What happens in the double-slit experiment with electrons if we place a detector behind one of the slits?
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1. we can then predict the precise point at which each electron will strike the screen.
2. we can know which slit each electron passes through, and the pattern on the screen is not affected.
3. we can know which slit each electron passes through, but the pattern on the screen is affected.
4. only half of the interference pattern on the screen will appear.