Video Transcript
A vertical mirror rests on a
flat horizontal surface. The orange arrow shows the
incident light ray on the mirror. Which of the following is
true? (A) The direction of the
reflected ray must be as shown by the red arrow. Or (B) the direction of the
reflected ray could be as shown by either the red arrow or the blue arrow.
To answer this question, we’ll
need to understand what the difference is between the red arrow and the blue
arrow. So let’s go over exactly what’s
being shown in the diagram. We know that this mirror stands
vertically and that this flat surface is horizontal. Notice this dashed line that’s
perpendicular to the face of the mirror. This line represents the normal
to the mirror, and it lies in the same horizontal plane as the flat surface. The orange arrow, which
represents the incident ray, and the red arrow also lie in the horizontal
plane. The blue arrow, however, looks
a little different. The blue arrow starts here at
the same point on the mirror as the red arrow, but it appears to rise up out of
the horizontal plane. That’s why we see this dotted
line. It’s there to show us that the
blue arrow rises up above the red arrow.
To see this better, we can draw
a side view of the diagram. This green line represents the
horizontal surface or the plane where the incident ray and the normal line
lie. This line represents the
mirror, which stands vertically. We can see that the blue arrow
starts here at the bottom of the mirror and points upwards by some amount. Therefore, the blue arrow does
not lie in the horizontal plane.
Now, the key to answering this
question is to simply recall that for light reflecting off a flat surface, the
incident ray, the normal to the surface, and the reflected ray must all lie in
the same plane. We know that the incident ray
and the line normal to the mirror lie in the horizontal plane. So the reflected ray also has
to lie in this plane. The red arrow lies in this
plane, but the blue arrow does not. Therefore, we know that the
blue arrow could not show the direction of the reflected ray. So answer option (A) is
correct. The direction of the reflected
ray must be as shown by the red arrow.
