In the double-slit experiment, when one of the slits is being observed, it introduces a measurement apparatus that interacts with the particles passing through that particular slit. This interaction disturbs the wave-like behavior of the particles and collapses their wavefunction into a localized state.
When the electron gun is aimed between the slits and the other slit is observed, the electrons will indeed fire at the wall behind the unobserved slit. The observation of one slit affects the behavior of the electrons, causing them to behave more like particles rather than exhibiting the interference pattern seen in the classic double-slit experiment.
To be more specific, when the electrons are fired from the electron gun, they will pass through the unobserved slit and continue to the wall behind it. The observation of the other slit, however, affects the path of the electrons that pass through it. This observation can be done by introducing a measurement device, such as a detector, which interacts with the electrons passing through that particular slit. The act of measurement disturbs the electrons' wavefunction, collapsing it into a specific position or momentum state.
As a result, the electrons that pass through the observed slit will no longer exhibit interference with the electrons passing through the unobserved slit. Instead, they behave more like particles and create a pattern on the wall behind the observed slit, similar to what you would expect from a particle fired through a single slit. The interference pattern is lost because the wave nature of the particles has been disturbed by the act of measurement.
It is important to note that the specific details and effects of the measurement process in the double-slit experiment can be complex and can depend on the specific experimental setup. However, the general principle remains that the act of observing or measuring one of the slits disrupts the interference pattern and alters the behavior of the particles passing through the observed slit.