Originally Posted by
tmorse
To better understand this question we must first assume some optical conventions…
1) standard lens position has plus surface first, followed by minus surface on back of lens.
2) all light rays move from left to right,
3) one of the 5 ray tracing rules is … any light ray running parallel to axis is refracted by the lens through the lens’s secondary focal point (F’) on the axis.
There is a primary focal point (F) and a secondary focal point (F’) in both plus and minus lenses. After passing through the plus or minus lens, where the emerging ray strikes the axis determines the position of the secondary focal point (F’).
Passing through the lens:
a) A light ray running parallel to the axis is converged by a plus lens to its secondary focal point (F’) on the axis to the right of the lens. Thus F’ is to the right of the plus lens.
b) A light ray running parallel to the axis is diverged by a minus lens away from the axis. But if we extrapolate back this diverging ray, it will strike the axis in front of the minus lens at the minus lens’s secondary focal pointy (F’). Thus F’ is to the left of a minus lens.
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