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. The ray passes through the center
of the lens. Which diagram correctly shows the
path of the ray after it passes through the lens? Is it diagram (A), (B), (C), or
(D)?
This question is asking about a ray
of light passing through the center of a concave lens. Remember that the direction of
light rays passing through the center of a lens always follows one important
rule. Any light ray that passes through
the center of a concave lens will exit the lens traveling in the same direction as
when the ray was incident on the lens.
Let’s look at a few examples. Here, we have a concave lens, with
the optical axis in green. Let’s draw one ray starting below
the optical axis and going through the center of the lens. The ray travels to the lens under
the optical axis, passes through the center of the lens, and continues traveling in
a straight line, which is now above the optical axis. Notice that the ray of light does
not change direction after passing through the lens.
Next, let’s draw a ray of light
starting above the optical axis and going through the center of the lens. The ray travels to the lens above
the optical axis, passes through the center of the lens, and continues traveling in
a straight line, which is now below the optical axis.
Now that we know that rays of light
traveling through the center of a concave lens do not change direction after
traveling through the lens, we can identify the correct answer. The light rays in (A), (B), and (D)
all change direction after traveling through the lens, so we know that these cannot
be correct answers. That leaves option (C) as the
correct answer, in which the ray does not change direction after traveling through
the center of the lens.
