In the double-slit experiment, the behavior of particles, such as electrons or photons, can exhibit wave-like properties. The crucial aspect of this experiment is the interference pattern that arises when particles pass through two slits and create an interference pattern on a screen placed behind them.
Whether or not an observer sees the slits while the particles are passing through them does make a difference in the outcome of the experiment. When an observer is present and actively tries to determine which slit a particle passes through, a phenomenon known as "the collapse of the wavefunction" occurs. This collapse refers to the transition of the particle's behavior from a wave-like nature to a particle-like nature.
The act of observation disturbs the delicate quantum state of the particle and collapses the wavefunction, causing the interference pattern to disappear. Instead, the particles behave more like classical particles and create two distinct, non-interfering bands on the screen, corresponding to the individual slits.
In summary, if an observer sees the slits while the particles are passing through them, it disrupts the wave-like behavior and destroys the interference pattern. The observer's presence and the act of measurement lead to a different outcome than when the experiment is conducted without observation.