Answers to additional concept questions:

- If you look at a window, you don’t see thin film interference, even though some light reflects off the front face of the window and additional light reflects off the back face. Why don’t you see interference patterns?

The thickness of the window is very large, probably tens of thousands of wavelengths. Also, the window isn’t perfectly flat, so slight variations in thickness are usually more than a wavelength. These two problems together make it very difficult for the path length difference to be uniform over a large enough region to observe interference patterns. This is in contrast to a film of gasoline on water, for example, where the surfaces are reasonably smooth and the film is very thin. In principle, if you had a thick piece of glass with extremely uniform thickness, you might be able to see “thin film interference,” but I am not sure that it is possible to have a thick piece of glass that is that uniform.

- In electron microscopes, electrons are used rather than light (photons). Why do electron microscopes have higher resolution than light microscopes?

Resolution is calculated from Rayleigh’s Criterion, which depends on the wavelength. For electrons, it’s the de Broglie wavelength that’s important. The de Broglie wavelength of the electrons used in electron microscopes is much shorter than the wavelength of light, so thus the resolution is much better, by Rayleigh’s Criterion. (The drawback of electron microscopes is that the object must be placed in vacuum, which precludes looking at things like living cells.)

- Describe a situation where electrons act like waves. Describe a situation where electrons act like particles.

Electrons act like waves if you send them through a 2-slit experiment, or shoot them through a very small hole. In electron microscopes, the resolution is limited by the de Broglie wavelength as described above, so that’s another situation where the wave-properties of electrons are relevant. However, in all three of those cases (2-slit experiment, diffraction through a small hole, electron microscopes) what is actually detected is the location of a particle-like electron impacting onto a screen, or a piece of film.