The double slit experiment shows that light is a wave, unless observed at the point where photons enter the slits. If observed there, the result of the experiment shows light as particles. What I want to know is: 1.) What instruments are used to observe the photons as it passes through the slits? 2.) Is it possible that the instruments themselves cause the interference, instead of just the fact that they are being observed? 3.) Why doesn't simply observing with the naked eye that light is going into the slits suffice to "change the physical state" of the light from waves to particles?