Video Transcript
The following figure shows rays
reflected from a convex mirror. Lines extended along the paths of
these rays meet on the opposite side of the mirror. What is the term for the point at
which these lines meet?
The question asks us to look at a
diagram of a convex mirror that is reflecting rays of light. When rays are reflected by a convex
mirror, they diverge as they travel away from the surface. The diagram shows that the
diverging rays will never cross each other on the side of the mirror from which they
were reflected.
Behind the mirror, however, the
paths of the reflected light rays have been extended, and these extended paths do
meet at a point. The point at which the extended
paths meet can be identified by imagining that the convex mirror is a concave
mirror. Recall that light rays traveling
parallel to the optical axis of a concave mirror will reflect from the mirror and
pass through the focal point of the mirror.
It is useful to recognize that the
position of the focal point of the mirror when it is being used as a concave mirror
is the same as the position of the focal point of the mirror when it is being used
as a convex mirror. We see that the dashed lines
showing the paths of the rays reflected from the convex mirror, extended behind the
mirror, have the same directions as light rays that would reflect from a concave
mirror after striking the mirror traveling parallel to the optical axis of the
mirror.
As we have shown that the paths
followed by the dashed lines have the same directions as reflected rays that pass
through the focal point of the mirror, the dashed lines must pass through the focal
point of the mirror. The term for the point where the
dashed lines meet is the focal point of the mirror.
