Video Transcript
The diagram below shows light rays
from the top of an object reflecting from a concave mirror. The object is closer to the mirror
than the focal point of the mirror. Two images are shown. Which image would actually be
formed?
In this question, we are given a
figure with a concave mirror, an object, light rays, and two possible images formed
by those rays. The focal point, center of
curvature, and optical axis of the mirror are also shown here. We are asked to figure out which of
the two images could be formed by this object and concave mirror. Before we start trying to figure
this out, let’s refresh our memories on concave mirrors and how they produce
images.
Recall that concave mirrors are
curved mirrors whose reflective surface is on the inside of curvature facing the
center of curvature and focal point. Also remember that concave mirrors
can produce both real and virtual images depending on where the object it’s
reflecting is located. Real images are formed in front of
the mirror, so image A would be a real image, while virtual images are formed behind
the mirror, so image B would be a virtual image. Real images are formed by concave
mirrors when an object is placed farther away from the mirror than the focal point
is. Virtual images are formed by
concave mirrors when an object is placed in between the focal point and the surface
of the mirror.
It is also important to remember
how reflected light rays create real and virtual images. A real image is formed when light
rays converge and meet at a point. A virtual image is created when
reflected rays diverge and spread out. If we trace these diverging rays
back through the mirror, their traced paths will meet at a point behind the mirror
at which the virtual image will be formed.
Now that we have refreshed our
memories on the properties of concave mirrors and how they produce images, let’s
take another look at our figure. First off, we can see that the
object is in between the mirror’s reflective surface and the focal point. We know that when this is the case,
a virtual image is formed. Also notice that the light rays
being reflected off the mirror are divergent and do not cross at any point after
being reflected, which also means a real image would not be formed.
Let’s trace them back and see where
they lead. We can see that image B is behind
the mirror, where virtual images are created and the diverging rays can be traced
back to meet at the top point of image B, forming the virtual image. So image B is the correct
answer.
