Shooting particles through one slit makes one band of particles at the back wall.
Shooting particles through two slits makes two bands of particles at the back wall.
Shooting a wave through one slit creates an intensity gradient on the back wall with the most intensity in the middle and weakening as you move to the sides.
Shooting a wave through two slits creates an interference pattern. Where the two waves (from the two slits meet) collide, they cancel each other out. So on the bback wall, you getting alternating intensity and nothing. Intensity appears when the two waves meet the wall. Nothing is when the waves collide and cancel each other out.
BTW, when I say particles, I mean everyday objects like marbles or cats or muffins or whatnot.
Now, when you shoot electrons* through a single slit, you get a single band on the wall. Normal.
But when you shoot electrons through two slits..you get an interference pattern.
How can pieces of matter create a wave-like intereference pattern?
So we observe the double slit filter. Let’s see which slit the electron choose to go through, right?
Wrong. With the addition of a mere observer, the electron changed the way it worked. When a measuring device was added to watch the double slit filter, the electrons created a normal, two band pattern in the back wall, not an intereference pattern like before.
The observer collapsed the wave function..simply by observing!
We did this experiment in grade 12 with red light and all kinds of fun stuff.
We did this experiment too (in 11th and 12th grade). First time I saw it, they were picking up chunks of my mind off the floor afterwards.