### Video Transcript

The diagram shows a convex
lens. Assume that light passes through
the lens in a horizontal direction. Which line shows the optical axis
of the lens?

Here we are given a figure of a
convex lens with five dotted lines through it. We are asked to figure out which
line shows us the optical axis of the lens. But first let’s recall some
information about convex lenses.

Notice that as viewed from the
side, the shape of a convex lens matches the same shape as the area where two
circles overlap. The centers of the two circles are
the centers of curvature for our lens. A center of curvature is the point
that is an equal distance to every point on the circle. This distance is known as the
radius of curvature. And we can see how the value of
this radius does not change, no matter which direction the radius is measured
in.

Now recall that the optical axis,
which is also called the principal axis, is the line that travels through the two
centers of curvature of the circles. Notice how the optical axis travels
straight through the center of our lens.

Now let’s compare the optical axis
we just found that travels through the two centers of curvature of our circles and
the lens to the figure that we were given in our problem. Looking through the five lines, we
can see that line 4 matches the optical axis because it passes through the center of
the lens, and it also passes through the points where the centers of curvature would
be of the circles that determine the shape of the lens. So line 4 is the correct
answer.