Video Transcript
Which one of the distances shown on
the diagram below is the focal length of a concave mirror?
On this diagram, we can see four
different distances. We want to know which one is the
focal length. So we need to recall that the focal
length of a concave mirror is the distance between the focal point and the back of
the mirror.
This point here shows the back of
the mirror, so the focal length should be measured from here. Therefore, we know that the
distances labeled one and four do not show the focal length.
To determine whether two or three
is correct then, we just need to remember where the focal point of a concave mirror
is. Let’s recall that parallel light
rays that get reflected from a concave mirror focus, or cross paths, at the focal
point of the mirror. In this diagram, we can see two
incoming parallel rays that get reflected from the surface of the mirror. The reflected rays cross paths at
this point here. So we know that this marks the
focal point. Therefore, we know that the
distance labeled three is incorrect. Rather, the distance labeled two
extends between the back of the mirror to the focal point.
Our final answer then is that
distance number two is the focal length of the concave mirror.
