# EM waves, double slit experiment, etc.

I’ve been given a HS Physics course to teach. I’m really a Chem guy, and with a little help I can do AP Chem, but I’m the only physical science teacher here (small school), and I just try to keep the Physics class learning something every day. It’s not up to the standards of the real physics people probably, but the district knew what they were getting.

Questions I can’t seem to find the answers to:

1. How wide apart can the slits be in the double slit experiment? I assume it depends on the size of the wave of the light or electron. But then how would you make slits so close together?

2. Is there any explanation for the idea of an electron interfering with itself and making an interference pattern on the far wall? If so, is there any that doesn’t use horrendous math?

3. Is there any explanation of how these particles have a wavefunction that collapses into only one state, or is this just one of those things unsolved, waiting for someone to explain and win a Nobel Prize? Is there any explanation of how observing causes the collapse?

4. I’ve been told that the mesh on a microwave window is to keep the photons of the microwaves inside, because the waves are too big to get through the holes. What I and my engineer dad can’t figure out is how to view the waves. In textbook diagrams, the waves are drawn as having the amplitude at 90 degrees to the direction of travel. but this wire mesh thing (and antenna vane length) seem to say that the wavelength can be seen to be traveling on their sides. Are the waves actually somehow in all directions somehow? Is there some direction that we could say the wavelength is not applicable?

They need to be so close together you can’t know which slit individual photons or electrons go through.

I’m not sure what you mean by how you make them so close together.

You’ll get constructive interference (a light spot) for all solutions to:
d sin(theta) = m lambda

Where d is the distance between the slits, theta is the angle of deflection, m is 0 or 1 or 2 or …, and lambda is the wavelength.

It has a wavelength. Everything does. Calculating the wavelength is easy. Explaining why is definitely not AP physics.

What’s the alternative to collapsing into one state? That’s what makes them particles.

The waves are changes in the electromagnetic field. You don’t want to try to actually make sense of that. The drawings are representations of the change in the strength of the fields, and the magnetic and electric fields are 90 degrees to each other and to the direction of travel, but you’ll only break your brain trying to make that “mean” something.

If the electron or photon is supposed to be able to interfere with itself, can I put the slits a meter apart? What’s the maximum distance?

I know it has a wavelength. Is there any kind of conceptual explanation of how these are able to interfere with themselves, or is that a point that’s still unknown?

Is there any conceptual explanation of how these seem to exist as a diffuse wave, then collapse when observed? Is there an explanation of how the observation could trigger that?

But they’re only perpendicular to the direction of travel? How then are they blocked by the metal screen according to their wavelength, and why do we make antennas with vanes appropriate for their wavelengths?

To get the classic interference pattern you assume that

wavelength << slit separation << distance from slits to screen

Young’s double slit experiment is easy to demonstrate and you should be able to find a lesson plan to suit your needs.

Yes, wave-particle duality, though be aware that this was a pre-quantum mechanical explanation and shouldn’t be mistaken for the full quantum explanation.

Wavefunction collapse is widely seen nowdays as a simplified description of the interaction between system, measuring appartus and observer, the problem is there is no universally-accepted explanation on how this interaction results in what we observe. This is part of what is called the quantum mechanical measurement problem and it would be an understatement to say that a widely-accepted solution to this problem would be huge.

Em waves are oscillations in the electromagnetic field and the amplitudes shown in the diagrams you have seen are electric and magnetic field vectors. The vectors can be seen as existing along the path of travel, but having both magnitude and direction.