In quantum mechanics, electrons are modeled as a "wavefunction" - an oscillatory function of both space and time, the squared magnitude of which corresponds to the probability of finding the electron at a given position/moment, if a measurement were to be made. When an electron is elastically scattered by a cryo-EM sample, this wavefunction is scattered in many directions simultaneously, and the amount of signal scattered at a particular angle is proportional to the spatial frequency spectrum of the sample. For example, if a sample contains a lot of signal at high spatial frequencies, then more of the electron wavefunction will be scattered at high angles. These scattered waves are subsequently focused by various lenses in the microscope to form an image, but because of their different scattering angles, they traverse a different total path length to arrive at the electron detector. These differences in path length manifest as phase shifts in the electron wavefunction, causing the scattered components of the wavefunction to interfere with each other. Depending on spatial frequency, this interference can be constructive, destructive, or inverting, and as a result, some spatial frequencies show up in the resulting image more strongly than others, and some are flipped in sign. This phenomenon is mathematically modelled by the microscope's Contrast Transfer Function (CTF). It is important to estimate and correct for the CTF during cryo-EM image processing, otherwise the achievable reconstruction will be of very limited resolution.