Video Transcript
Below is a series of ray diagrams
for convex mirrors. All four mirrors have identical
incident light reflecting off their surfaces. This is shown by the yellow
lines. Which of the four mirrors has the
greatest focal length?
To answer this question, we need to
understand the definition of the focal length of a spherical mirror. The focal length is the distance
between the point where the reflected light rays meet, called the focal point, and
the surface of the mirror. However, since these are convex
mirrors, the reflected light rays do not meet. Instead, the focal point is the
point where they would meet if we traced the direction of the reflected light rays
back through the mirror.
Now that we have defined the focal
length, we can compare these distances on each of the four ray diagrams and find
which distance is the greatest. We can clearly see that mirrors one
and two do not have the largest focal length. Hence, these options cannot be the
correct answer. Comparing mirrors three and four,
we see that mirror three has the larger focal length. Hence, the correct answer is that
mirror three has the greatest focal length.
