Video Transcript
Each of the following diagrams
shows a ray entering a thin concave lens. The point marked P is the focal
point of the lens. Before entering the lens, the ray
is parallel to the optical axis of the lens. Which diagram correctly shows the
path of the ray after it passes through the lens?
Okay, so this question shows us
five different diagrams in which a light ray passes through a concave lens. We’re told that the point marked P
in each of the diagrams is the focal point of the lens. And we’re also told that before it
enters the lens, the ray is parallel to the optical axis. We can recall that there is a rule
for concave lenses that says any light ray, which is initially parallel to the
optical axis, has its direction changed by the lens so that its new direction after
the lens is on a line that goes through the lens’s focal point.
If we look at diagram A, we can see
that the incoming ray does indeed have its direction changed by the lens. We can also see that when this new
ray direction is extended back before the lens, the extended line goes through the
point marked P, which we know is the focal point of the lens. This means that the ray’s path as
shown in diagram A does follow this rule. Let’s now check out the other
diagrams to make sure that we’ve got the right answer.
In the diagram labeled B, the
direction of the ray is completely unchanged by the concave lens. But we know that its direction
should be changed so that its new direction is on a line through this point marked
P. So, this diagram cannot be
correct. Looking at the diagrams labeled C,
D, and E, we see that in each case the ray’s direction is changed by the concave
lens. When we extend the new ray
direction in diagram C back before the lens, it’s clear that it doesn’t go through
the point marked P. The same thing is true for the rays
shown in diagrams D and E. So, in these three diagrams, the
new ray direction is not on a line that goes through the focal point. So, diagrams C, D, and E cannot be
correct.
This leaves us with diagram A,
which we already saw shows the ray’s direction getting changed so that it’s on a
line through the focal point. And so, our answer to the question
is that the diagram that correctly shows the path of the ray after it passes through
the lens is the diagram labeled A.
