Video Transcript
The diagram shows five light rays
that will pass through a thin convex lens. The center of the lens is indicated
by a black dot. How many light rays will not change
direction as they pass through the lens?
To answer this question, we must
remember two basic and very important rules: first, that any light ray that passes
through the center of a convex lens does not change direction; next, that any light
ray that is parallel to, but not along, the optical axis will change direction when
it passes through a convex lens so that the ray passes through the focal point of
the lens that is on the opposite side of the lens to the side that light enters the
lens from.
Looking now at the diagram, we can
see that there is only one ray that is not directed at the center of the lens, which
is represented by the black point. This is the ray that is traveling
to the right and above the optical axis. This ray will have to change
direction to pass through the focal point on the opposite side because the ray is
parallel to the optical axis and the lens is convex.
All the other rays are directed
toward the center of the convex lens and therefore would not change their path. There are five rays in total. Only one of these rays will change
direction. And so four rays will not change
direction. Therefore, the answer to the
question is four light rays.
