Video Transcript
The diagram shows five light rays
that will pass through a thin concave lens. The center of the lens is indicated
by the black dot. How many light rays will not change
direction as they pass through the lens?
In this question, we’ve been given
a concave lens. We know it’s concave since it’s
thinner in the center than at the edges. Let’s recall two important rules
for ideal concave lenses. First, any light ray that passes
through the center of a concave lens does not change direction. Next, any light ray that is
parallel to, but not along, the optical axis will change direction when it passes
through a concave lens such that the refracted ray appears to have passed through
the focal point when viewed by an observer on this side of the lens.
Looking at the given diagram, we
can see that there’s only one ray that’s not directed toward the center of the
lens. That’s this ray here, which is
parallel to, and above, the axis of the lens. The other four rays are directed to
the center of the lens, and therefore, after passing through the lens, they will not
change direction.
Our final answer to this question
then is that four rays will not change direction as they pass through the lens.
