# Why can't light pass through a gold foil, but alpha particles can?

Rutherfords experiment showed that most of the alpha particles passed through a thin gold foil undeflected. So why can't light pass through the foil, which is supposedly thinner than the heavy alpha particles?

• Search for "effective skin depth".
– A.K.
Aug 16 '18 at 3:23

In truth, comparing the mean free path or collision cross-sections of photons and alpha particles through a gold sheet is somewhat more complicated than just saying alpha particles should penetrate less because they're massive, so it would be conceivable for a film of appropriate thickness to block photons yet still allow alpha particles through. A very coarse, hand-waving explanation is that photons are absorbed in the electrosphere of a gold atom, while an alpha particle is only significantly pushed away if it bangs almost straight into the nucleus, a region composing approximately $10^{-15}$ of the volume of the atom.