Video Transcript
The ray diagram below shows
different paths taken by parallel rays after they reflect off a concave mirror. Which of the paths is correct? (A) Path 1, because a concave
mirror will focus the incident light at a point. Or (B) path 2, because a concave
mirror will not focus the incident light at a point.
So, in this question, we’ve been
asked about the path traveled by a ray of light after reflecting off the surface of
a concave mirror. We’ve also been given two possible
paths that the ray could take after being reflected: path 1, which passes through a
point on the optical axis, and path 2, which moves away from the axis.
Let’s recall that a concave mirror
reflects any ray that is initially traveling parallel to the optical axis so it ends
up passing through the focus of the lens. The focus, or focal point, is a
point on the optical axis. And for a concave mirror, this is
between the center of curvature and the reflecting surface. In fact, the focal distance for a
spherical concave mirror is exactly half the distance between the mirror and the
center of curvature and is the point where the rays reflected off the mirror are
focused.
The main takeaway for us here
though is that since we have a ray of light initially moving parallel to the optical
axis, the concave mirror will reflect it so that it passes through the focal point
on the optical axis. For this reason, path 1 shows the
correct path taken by the ray of light. Therefore, we choose answer option
(A) as our final answer, which says “Path 1, because a concave mirror will focus the
incident light at a point.”
