Video Transcript
Below is a ray diagram for a convex
mirror. Which one of the five locations
along the optical access represents the focal point of the spherical mirror?
Since this mirror is convex, that
means an observer would be on this side of the mirror. Since the mirror is spherical, that
means its surface is part of a larger sphere. In this example, we have five
locations, one, two, three, four, five, along what is called the optical axis. That axis follows this line that
goes right through the center of the mirror. We want to figure out which of
these five locations represents the focal point of the mirror. Notice that the diagram shows these
three parallel rays of light running into the mirror. The top ray reflects off in this
direction, the middle ray reflects straight back along the path it came, and the
bottom ray reflects off the mirror like this.
To find the location of the focal
point, we’ll trace these three reflected rays backward. The top ray is traced backward like
this, the middle ray like this, and the bottom ray this way. We’ve only traced back far enough
to find the point where the rays meet, where they cross. That point is here, and this is the
focal point of the mirror. We see this is at the location
marked four. Location four is the mirror’s focal
point.