- #1

center o bass

- 560

- 2

*there are two foci on each ray of a rectilinear congruence.*

Here a rectilinear congruence of light rays is defined as a collection of

**straight**light rays such that for each point in space there is one ray of the congruence going through that point.

A foci is defined as a point on a specific ray such that the distance to a neighbouring ray vanish to first order.

Question: Intuitively, I would think that straight rays would just converge at a single point, not two. How can it be two places on a single straight ray where it get "close" to its straight neighbouring rays?