What I don’t understand is how it was possible (whether in the electron or photon versions) to see interference on the screen. Wouldn’t the width of the interference lines be somewhere close to the wavelength of light. Wouldn’t that be almost impossible to see?

The distance between the slits and from slits to projection surface are the other two big factors in the frequency of the interference.

IIRC they use something like a photographic film as the target. Each individual photon/electron cannot be discerened but they let the experiemtnexperiment run awhile and eventually they all add up. When the film is developed you get the light and dark banding. The photons/electrons do not all hit in precisely the same spot every time. Heisenberg Uncertainty Principle here (dunno). They get scattered about a bit so you get broader lines than one the width of a wavelength of light.

Doubtless these days there are more hi-tech ways to go about it (an electronic detector of some sort that registers a photon/electron impact and displays the results on a monitor for instance…dunno).

If the distance between slits is **a** and the distance to the screen is **L**, and the wavelength of light is **Lambda**, and the distance between the the center of the pattern and the first minimum is **d**, then the first minimum occurs at an angle theta so that **a** sin (theta) = **Lambda**, meaning that there’s a half-wave difference between the two paths (assuming **L** is a lot bigger than **a**). But we also have **L** sin (theta) = **d**, so **d** = **Lambda (L/a)**. Since we’ve assumed **L** is a lot bigger than **a** (and we can always make it larger by moving the screen farther away), the widths of the bars are or can be made many times larger than the wavelength of light.

There are a lot of ways to do this experiment, using photographic film, your eyes, detectors, cameras, etc. I’ve done it with interfering ions using a particle-beam apparatus and a detector.

Here’s a similar experiment, using the rulings on a metal ruler as an impromptu diffraction grating: