Video Transcript
Each of the following diagrams
shows a ray entering a thin convex lens. The point marked 𝑃 is the focal
point of the lens. Before entering the lens, the ray
is parallel to the optical axis of the lens. Which diagram correctly shows the
path of the ray after it passes through the lens? Is it diagram (A), diagram (B),
diagram (C), or diagram (D)?
In this question, we’ve been asked
to work out how a convex lens will alter the path of a ray of light. We’re told that the ray of light is
traveling parallel to the optical axis before it enters the lens. So, we know that we have this
situation here. To solve this problem, we must
first remember the two rules for what happens to the direction of light rays when
they pass through convex lenses.
The first rule is that if the ray
passes through the center of the lens, then it doesn’t change direction. When the light ray leaves the lens,
it is traveling in exactly the same direction as when it entered the lens. This is true no matter what the
initial direction of the light ray. However, in this question, the ray
of light isn’t passing through the center of the lens. This means that we need to use our
other rule for convex lenses.
The second rule says that any ray
of light traveling parallel to the optical axis before passing through the convex
lens will change direction so that it passes through the focal point of the
lens. The focal point is the place where
light rays meet after they’ve passed through the lens, so long as they were all
traveling parallel to the optical axis when they entered the lens. Essentially, the convex lens
focuses the light rays at this one point, which is along the line of the optical
axis.
Knowing this key piece of
information, we can now answer this question. We need to look at the options
we’ve been given and ask ourselves which diagram correctly shows this rule for
convex lenses. In each of the options (A), (B),
and (C), the light ray does not pass through the focal point labeled 𝑃. The only diagram where the ray
passes through the focal point is option (D). So, the correct answer must be
option (D).
