Figure 3.

A 1D object with two signal sources at two different locations (red and blue in the top row) gives rise to two signals oscillating at different frequencies (second row). These signals are sampled at a high rate (third row left) and at a lower rate (third row right), where the sampling time is indicated by vertical dotted lines. The points sampled are denoted as triangles; at some sampling times, the two signals appear the same (black triangles). If the signals are sampled at a high enough rate (left bottom), the frequencies can be distinguished from one another and the two locations can be resolved. If the signals are sampled too slowly (right bottom), or undersampled, the two frequencies appear the same at these sampled points and the two locations cannot be distinguished.