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 the ray enters the lens, it
is parallel to the optical axis. It passes through the center of the
lens. Which diagram correctly shows the
path of the ray after it passes through the lens? Diagram (A), diagram (B), or
diagram (C).
This question is asking us about a
light ray that is initially parallel to the optical axis and passes through the
center of the lens. In order to answer this question,
we must recall a rule for light rays passing through a concave lens. The rule states that any light ray
that passes through the center of a concave lens will exit the lens traveling in the
same direction as when it was incident on the lens.
The problem states that the ray of
light passes through the center of the concave lens. So we know that the ray of light
will not change direction after passing through the lens. Now we just need to look at each of
these options and identify which ray diagram correctly shows the ray traveling along
the same direction after passing through the lens.
First, let’s look at option
(A). If we trace the direction of the
light ray exiting the lens backward through the lens without changing direction, as
shown by the blue dashed line, then we can see that it does not match the direction
of the ray as it enters the lens. Therefore, we know that this cannot
by the correct option because the ray exits the lens traveling in a different
direction then that it traveled in before entering the lens. This means we can eliminate answer
choice (A).
Next, let’s look at option (C). Again, we can trace the direction
of the light ray after exiting the lens backward through the center of the lens. And again, we see that the trace
does not match the direction of the ray entering the lens. So option (C) cannot be the correct
option either because the ray exits the lens traveling in a different direction to
that it traveled in before entering the lens. We can therefore eliminate answer
choice (C).
That leaves us with option (B). Let’s again trace back the
direction of the light ray after traveling through the center of the lens to verify
that this is the correct answer. When tracing this exiting ray back
through the center of the lens, it matches perfectly with the ray of light entering
the lens. This ray exits the lens traveling
in the same direction as that it traveled in before entering the lens. So it follows the rule for light
rays passing through the center of a concave lens. And so, we have confirmed that
option (B) is the correct answer.
The diagram which correctly shows
the path of the light ray after it passes through the concave lens is diagram
(B).
