Video Transcript
Which of the following statements
correctly describes what is meant by the term “converging light rays”? (A) Light rays are converging if
they are parallel. (B) Light rays are converging if
they get further apart as time passes. (C) Light rays are converging if
they get closer together as time passes and meet at a point.
Here, since we’re talking
converging light rays, we might recall that convex lenses are sometimes called
converging lenses, since they can make light rays converge. Let’s draw a diagram to show what
this looks like.
Here is a convex lens. Specifically, let’s look at what
happens when parallel light rays enter the convex lens. Notice that these parallel light
rays are traveling to the right. And after the rays exit the lens,
they get closer together and eventually meet at a point. This is what is meant by the term
“converging light rays.”
Now, option (A) says that parallel
light rays are converging. We know this isn’t correct. In our diagram, the light rays are
parallel at first, but when they exit the lens, they’re no longer parallel because
they converge. Converging light rays meet at a
single point, and we know that parallel lines do not meet. Therefore, we should eliminate
answer choice (A).
Next, option (B) says that
converging rays get farther apart as time passes. This can’t be true, since
converging rays must eventually meet at a point. And in order for this to happen,
the rays must get closer together as time passes, not farther apart. Let’s cross out this answer choice
as well.
Option (C) says that converging
rays get closer together as time passes. This agrees with our understanding
of what it means for rays to converge. We see the rays converging, or
getting closer together, right after they exit the lens, and this allows them to
come together at this point here. In fact, this is exactly what a
convex or converging lens is designed to do. Therefore, we know that answer
choice (C) is correct. Light rays are converging if they
get closer together as time passes and meet at a point.
