Video Transcript
Which of the following statements
correctly describes a convex lens? A) A convex lens is thicker than a
concave lens. B) A convex lens increases in
thickness from its center to its edges. C) A convex lens decreases in
thickness from its center to its edges. D) A convex lens has uniform
thickness.
So to answer this question, we’re
trying to recall what a convex lens actually is. And to answer this, we need to know
that the word convex refers to any surface that curves the same way as the outside
of a sphere. In other words then, if this is our
sphere and we place an observer on the outside of the sphere. So this is the eye of the
observer. Then we see that the surface of the
sphere starts far away from the observer. Then comes towards the observer and
then bends away from the observer again. And a convex lens simply has that
same convex behavior on both sides of the lens. In other words, a convex lens is
convex on one side. And it’s convex on the other as
well. Sometimes, therefore, it’s known as
a biconvex lens, bi meaning two showing that it’s convex on the left and on the
right.
So looking at the first possible
answer, option A. This says that a convex lens is
thicker than a concave lens. Now that’s not necessarily
true. Firstly, we recall that concave is
any surface that bends like the inside of a sphere. So if we place our observer here,
then they see the inside of the sphere as starting near them at the top. Then curving away from them and
then curving back towards them. And so a concave lens, or biconcave
lens, is one which is concave on one side and concave on the other. And the way that we’ve drawn this
concave lens it is thinner than the convex lens. However, this is also a concave or,
rather, a biconcave lens. And yet it’s much thicker than our
convex lens. So it’s not necessary for a convex
lens to be thicker than a concave lens. Hence, option A is not what we’re
looking for.
Option B then, a convex lens
increases in thickness from its center to its edges. Okay, so if we start at the center,
we see that it’s got a fairly large thickness. And then as we go towards the
edges, or as we go higher up or lower down, we see that the thickness actually
decreases. And hence, option B is not the
answer we’re looking for. Option C says the opposite. A convex lens decreases in
thickness from its center to its edges. And we’ve just seen that this is
true. If we start at the center, the
thickness is large. And as we go further towards the
edges, the thickness decreases. So it looks like option C may be
our answer. But if we quickly look at option D,
this one says that a convex lens has uniform thickness. And we’ve clearly seen that that’s
not true. Once again, the convex lens is
thick in the middle. And it’s thin at the edges. Therefore, it cannot have a uniform
or same thickness all the way along its length.
Hence, we’ve found our answer. A convex lens decreases in
thickness from its center to its edges.
