Video Transcript
Which of the following diagrams
shows what happens when parallel light rays pass through a thin concave lens? (A), (B), (C), (D), or (E).
This question is asking us to
recognize the effect that concave lenses have on parallel rays of light. Before we answer this question,
it’ll be useful to recap what we know about concave lenses. A concave lens looks like this. When light reaches the lens, the
lens changes the path of the light. When parallel rays of light reach a
concave lens, the lens causes the rays of light to spread out. Once they’ve passed through the
lens, they’re no longer parallel to each other. And the distance between each ray
of light increases as the distance of a light from the lens increases.
When rays of light spread out from
each other, we say they are diverging. Because a concave lens causes light
rays to diverge, a concave lens is sometimes called a diverging lens. If we look at these diverging light
rays, we might notice something interesting. If we trace each of these rays
backwards, ignoring the lens, it appears as though all of the rays come from a
single point. We call this point the focal
point. Note that the actual light rays
don’t all physically pass through this point. It just appears that they come from
this point because of the way the lens has caused the rays to spread out.
This diagram actually sums up the
most important things we need to remember about concave lenses. Concave lenses cause parallel rays
of light to diverge. When this happens, it appears that
the rays of light are coming from a point behind the lens, called the focal
point.
With this in mind, we are ready to
tackle this question. Let’s start with option (E). In this diagram, it looks like the
lens isn’t doing anything at all. The light rays are parallel before
they enter the lens, and they’re parallel after passing through the lens. This is the same path that the
light rays would take if the lens wasn’t there. So option (E) cannot be the correct
option.
Next, let’s look at options (C) and
(D). Option (C) shows all of the light
rays bending downwards by the same amount, and option (D) shows all of the light
rays bending upwards by the same amount. Although the light rays have
changed direction, because each ray has changed direction by the same amount, the
light rays are still parallel after passing through the lens. They’re certainly not diverging in
the way that we would expect if they’d passed through a concave lens. So these cannot be correct
answers.
Next, let’s think about (B). At first glance, this might seem
like a good option. If we look here, we can see that
the rays are spreading out, after having come from a point. However, option (B) is not the
correct answer. To understand why, we have to look
at what’s happening at the surface of the lens. If we look at this part of the
diagram, we see that the lens is actually causing the light rays to move towards
each other and not to spread out. These rays then meet at this point,
in front of the lens. But this isn’t what concave lenses
do either. Concave lenses cause the rays to
spread out so it appears as if they came from a point behind the lens. So option (B) is not correct.
That leaves us with option (A). Option (A) shows the light rays
spreading out after they pass through the lens. If we trace these rays back, it
looks as though the rays have come from a point behind the lens. This is exactly what we would
expect to see when parallel light rays pass through a concave lens. So option (A) is the correct
answer.
