Quantum electrodynamics facts for kids
Quantum electrodynamics (also called QED for short) is the quantum theory of electricity and magnetism. QED describes how electric and magnetic forces work on the scale of things smaller than atoms. QED also describes how single photons of light behave. QED is one of the most welltested theories in physics.
Images for kids

Feynman (center) and Oppenheimer (right) at Los Alamos.

Feynman replaces complex numbers with spinning arrows, which start at emission and end at detection of a particle. The sum of all resulting arrows gives a final arrow whose length squared equals the probability of the event. In this diagram, light emitted by the source S can reach the detector at P by bouncing off the mirror (in blue) at various points. Each one of the paths has an arrow associated with it (whose direction changes uniformly with the time taken for the light to traverse the path). To correctly calculate the total probability for light to reach P starting at S, one needs to sum the arrows for all such paths. The graph below depicts the total time spent to traverse each of the paths above.
See also
In Spanish: Electrodinámica cuántica para niños