Video Transcript
Which of the following is the
correct definition of the focal length of a lens? (A) The focal length of a lens is
the distance between the center of the lens and one of its centers of curvature. (B) The focal length of a lens is
the distance between its two foci. (C) The focal length of a lens is
the distance between the center of the lens and one of its foci. (D) The focal length of a lens is
the distance between its two centers of curvature.
In this question, we are asked to
choose one of four statements that correctly describes the focal length of a
lens. Let’s remember what the focal
length is with the aid of a diagram. Let’s draw a thin, convex lens. When we’re thinking about lenses,
there are several important things to remember. The side view of a convex lens
should look like the overlap between two circles. The centers of these circles are
called the centers of curvature of the lens, and the straight line that joins them
is called the optical axis.
When parallel rays of light pass
through a convex lens, the lens changes the direction of the rays so that they
converge or get closer together. Having passed through the lens, the
light rays meet at a point on the optical axis, called the focal point. Because light can pass through the
lens in either direction, the lens actually has two focal points, or foci, one on
each side.
So, what is the focal length
then? The focal length is the distance
from the center of the lens, which we’ve marked with this pink dashed line, to one
of the focal points. This distance needs to be measured
along the optical axis. Now that we know what the focal
length is, we can choose the correct answer from the list of options.
We can rule out options (A) and (D)
straightaway because these both mention the centers of curvature of the lens. Although the lens does have two
centers of curvature, neither of them is used to determine the focal length of the
lens. So, these options cannot be
correct.
Now, let’s look at option (B). The focal length of a lens is the
distance between its two foci. This may sound feasible, because we
know that the focal length of a lens depends on the position of the focal point. However, the focal length is not
measured between the two foci. The focal length is measured
between the center of the lens and one of the focal points. So, option (B) cannot be
correct.
This leaves us with option (C). The focal length of a lens is the
distance between the center of the lens and one of its foci. This is correct, so option (C) is
our final answer.
