While reading Feynman's 6 Easy Pieces, I see he talks about bouncing photons off of the electrons that are passing through two slits. According to Feynman, when a photon strikes an electron near one of the slits, it bounces (sometimes back at the observer) and registered as a sharp point of light for the observer. In the process, the electrons momentum is disturbed to a degree that is inversely proportional to the wavelength of the light used. In the next experiment, light of a much longer wavelength is used (in order to strike the electrons with a minimal change in electron momentum). Feynman describes how the photon will bounce to the observer, but will cause a blurry area of light for the observer, rather than a well defined point of light. My questions: Why do longer wavelengths have this effect of blurring/uncertainty? And what are we finding out when we use longer wavelengths? The momentum of the electron? How do we find the momentum of an electron simply from bouncing blurry light off of it?