### Video Transcript

A spherical mirror has a focal
length of 860 millimeters. What is the distance, in meters,
between the point on the surface of the mirror that the optical axis passes through
and the focal point? (A) 860 meters, (B) 0.43 meters,
(C) 0.86 meters, (D) 430 meters, (E) 1.72 meters.

In this question, we are
considering a spherical mirror, so we’ll start by drawing a diagram of one. We’ve been told that the focal
length of this spherical mirror is 860 millimeters. Let’s recall that the focal length
of a spherical mirror is the distance between the focal point and the nearest point
to it on the surface of the mirror. In other words, this is the
distance between the focal point and the point on the mirror surface that the
optical axis passes through. Now this might sound familiar.

The question actually asks us to
find the distance between the focal point and the point on the mirror surface that
the optical axis passes through. We’ve just seen that this distance
is the focal length. We can imagine then that the answer
to the question must be 860 millimeters. However, we need to be careful
here. The question asks us for this
distance in meters, and not in millimeters. To convert this distance to meters,
let’s recall that one millimeter is the same as 0.001 meters. So 860 millimeters is simply 860
multiplied by 0.001 meters. This ends up being 0.860 meters, or
0.86 meters. From our answer options, we can see
this matches option (C). Hence, we choose this as our final
answer.

The distance between the point on
the surface of the mirror that the optical axis passes through and the focal point
is 0.86 meters.