Video Transcript
Does the following figure correctly
show the reflection of light rays from a plane mirror producing an image of the
object in front of the mirror?
This question shows an object in
front of a plane mirror. Two parallel light rays from two
different points on the object are incident on and then reflected from the
mirror. The question also shows an image of
the object. The question asks whether the paths
of the light rays and the image are both correctly shown. To answer the question, it is
necessary to check both whether the light rays are correctly shown and whether the
image is correctly shown.
Let us first consider the paths of
the light rays. When light reflects from a plane
mirror, the angle of incidence, which is the angle between the incident light ray
and the normal line to the surface of the mirror, is equal to the angle of
reflection, which is the angle between the reflected ray and the normal line to the
surface of the mirror. This mirror is vertically oriented,
so the normal line to the surface of the mirror is horizontal.
We can see that for both light rays
from the object, the magenta angle below the horizontal is equal to the blue angle
above the horizontal. This means that the paths of the
incident and reflected light rays are drawn correctly.
Let us now consider whether the
image is drawn correctly. One way to check if the image is
drawn correctly is to recall that the distance of a point on the object from the
mirror is equal to the distance from the corresponding points on the image to the
mirror. The points on the outline shown are
the same distance from the image as the corresponding points on the object.
Another way to check if the image
is drawn correctly is to recall that light rays appear to travel in straight
lines. When an image is seen, it will have
a position and shape consistent with the light rays from the image having apparently
traveled in straight lines. We can then find the apparent
positions of the points on the image by extending the reflected light ray paths
behind the mirror.
Using both of these ways of
checking, we see that they give the same shape and position of the image. We also see that this shape and
position is not that of the image given in the question. We conclude then that the correct
answer is no. The figure does not correctly show
the reflection of light rays from a plane mirror producing an image of the object in
front of the mirror.
