Video Transcript
Which of the following diagrams
shows what happens when parallel light rays pass through a thin convex lens? (A), (B), (C), (D), or (E).
In this question, we are being
asked to identify the diagram that correctly shows the path of light rays that pass
through a convex lens. We are given five diagrams where
parallel rays of light are incident on convex lenses. We’re being asked what happens to
the light rays next when they leave the lens. Let’s first remember what we know
about convex lenses and draw a diagram showing their key features.
First, let’s draw in the optical
axis. The optical axis is an imaginary
line that passes through the widest part of a convex lens. When rays of light entering the
lens are parallel to the optical axis, like this, the lens causes the rays to
converge. This means that the light rays
change direction and move closer together. The light rays meet at a single
point on the optical axis of the lens, called the focal point.
Now let’s compare this to the
diagrams we have been given in this question. We can see that in all of these
diagrams, parallel rays of light are entering the lens.
In option (A), the lens has no
effect on the direction of the light rays, with all the rays remaining parallel
after they leave the lens. So, this cannot be the correct
answer.
In option (B), the lens changes the
direction of the light so that the light rays converge at a single focal point along
the line of the optical axis. This matches our earlier
description of what should happen when light passes through a convex lens. So, option (B) is a good candidate
for the correct answer.
In option (C), the lens changes the
direction of the rays so that they move further apart and spread out. This is also called diverging. These rays will never meet at a
single point, so this cannot be the correct answer.
In option (D), the lens changes the
direction of all the light rays by the same amount. Even though the light rays have
changed direction, they are still parallel after they’ve passed through the
lens. Again, these light rays will never
meet at a single point. So, this cannot be the correct
answer.
Finally, in option (E), the lens
causes the light rays to move closer together. However, the rays do not all meet
at a single point. Three of the rays meet here, but
this fourth ray doesn’t. There is no single point that all
the rays pass through. So, this diagram cannot correctly
show the effect of a convex lens.
That leaves us with option (B). This correctly shows the convex
lens causing the rays to converge at a single point. So, option (B) is the correct
answer.