# Arranging for beams to meet

Physics Narrative for 14-16

## Physical arrangements used to make beams meet

Radiations travel in beams: the power in a single beam, often related to the number of photons a second (emitted or absorbed), is a truly useful quantity. Quite often it is enough to describe the possible effects of the energy being shifted. However, when two beams meet, you observe significant new phenomena. Because, in the simpler cases, these depend on arranging for two separate beams to meet, it's a good idea to have these situations clear before attempting an analysis, concentrating on how we arrange for beams to meet.

After that it's worth extending the idea to situations where you cannot readily describe the situation in terms of two beams, but where two distinct vibrations do contribute: that is, where we can find two or more significant paths from the source to the detector.

Here are the four most common different ways of arranging for two beams to meet.