Video Transcript
Fill in the blanks. According to the Huygens wave model
of electromagnetic radiation, an electromagnetic wave originates at a blank and
moves in blank from the blank. (A) Surface, one direction,
surface. (B) Point, all directions,
point.
Our answer options give us two
possible explanations for what’s called the Huygens wave model of electromagnetic
radiation. This wave model actually applies
not just to electromagnetic radiation but to all waves.
Say that we have a series of wave
fronts like this propagating to the right. Note that these wave fronts are
flat; they’re not curved lines but are straight. This means that whether we model
the wave fronts as originating from a surface and moving in one direction from that
surface, say forward in the direction of our waves propagation here, or whether we
model the electromagnetic waves as originating from a point, say, for example, at
this point here on this wave front, and then propagating out in all directions from
that point, so long as we assume that all points along the wave front are equally
sources of waves that move out in all directions, either one of these two different
wave models can explain the wave propagation we see here.
However, let’s say that we pass
these straight wave fronts through a small opening in a wall. When that happens, the pattern of
wave fronts on the other side looks something like this. With these curved wave fronts, we
can no longer say that both answers are valid in explaining how the wave moves
along. If we consider our first answer
option that this wave model describes waves originating at a surface, then we could
imagine, say, this surface giving off another wave front. But the size of that wave front
would be limited to the size of this surface. Even if that wave front did then
move to the right as we see the wave fronts moving here, it wouldn’t be able to
create the next wave front we see in the series. After all, these wave fronts we see
are getting longer and longer.
What can produce these successive
wave fronts though is if each point on each wave front is a source of other
waves. If these waves then propagate out
in all directions and interfere with one another, then in that case we can get the
wave fronts we see here, which are broadening or widening out, we could say it, from
this opening. This shows us that of the two
descriptions given here, it’s description (B) that accurately tells us about the
Huygens wave model of electromagnetic radiation. Our correctly completed sentence
reads “According to the Huygens wave model of electromagnetic radiation, an
electromagnetic wave originates at a point and moves in all directions from the
point.”
