Video Transcript
The diagram shows many light rays
and a thin convex lens. How many rays will pass through the
left focus?
To answer this question, we must
remember two basic and very important rules: first, that any light ray that passes
through the center of a convex lens does not change direction; next, that a light
ray that is parallel to, but not along, the optical axis will change direction when
it passes through a convex lens so that the ray passes through the focal point of
the lens that is on the opposite side of the lens to the side that light enters the
lens from.
Analyzing the diagram, we can see
that there is no ray directed toward the center of the lens. To find rays that would pass
through the left focus, we must instead look for the rays that are parallel to the
optical axis. We know that the optical axis
passes through the two focal points. In the diagram in this question,
the optical axis is therefore a horizontal line. This means that we should look for
horizontal light rays on the right-hand side of the lens.
We can see that there are three
rays that fit this description. One of the rays is traveling to the
right, however. So that ray will not pass through
the left focus. Therefore, the answer to the
question is two rays will pass through the left focus.
