in this video we're going
through the air now they move really
really fast and in fact a sound wave
would take less than a thousandth of a
second to go from one side of the mirror
to the other and that's way too fast to
see so we're going to use the
stroboscopic effect to try to phrase
these waves or give us the illusion that
they're frozen in space you've probably
seen this kind of effect before if
you've especially if you've used the
stroboscope like if you have a rapidly
spinning wheel if you flash a bright
light on it every time it makes exactly
one revolution and it's off the rest of
the time then it gives the empirics just
motionless because you see it in that
same position every time so we're going
to do something very similar in this
setup the transducer here is producing
sound waves and every time it produces a
wave we're going to flash a light on for
just a few microseconds and that will
effectively freeze that wave in space
but you're not going to see just one
wave frozen in space because this thing
produces 28,000 waves every second so
you will see a whole series of them
frozen in space so let's have a look
alright I'm going to turn on the sound
waves the transducer and I'm going to
wear a hearing protection even though I
can't hear the sound
so there you see the waves frozen in
space I've also taken this ruler and put
tape marks on at every one point two
centimeters I'm going to hold that up to
the transducer and you can see that the
distance between the waves which is the
wavelength is 1.2 centimeters so knowing
the wavelength knowing that it's 28,000
waves per second you can calculate the
speed of these waves now since we're
trying to visualize waves traveling
through space we don't really want to
see them frozen so let me turn on the
transducer again what we'd like to do or
what like to see is them traveling
forward in space and we can make that
happen by strobing the light at a
frequency that's a little bit less than
the frequency of the waves being
produced in doing so what's going to
happen is that every time the light
comes on because it's a little bit
slower than the waves the wave will have
progressed forward in space just a tiny
bit so it'll give the illusion that the
wave is moving forward so Allen is going
to adjust the frequency of the strobe
just to be a few Hertz less than the
actual frequency of the waves and you
can see that this gives us the illusion
that they're actually progressing
forward in space which is kind of the
way we like to see waves going if on the
other hand Allen adjusts the strobe
frequency to be a little bit greater
than the frequency of the waves then the
wave will not have traveled quite as far
as every time that the light comes on
and that gives the illusion that is
actually moving backwards which is
absolutely not
happening it's just an illusion so Ellen
is going to adjust it so that the waves
look like they're progressing forward
seeing the waves move forward we can now
show you some of the characteristics
that that are common to all waves here's
a piece of plastic which I'm going to
use as a reflector and put it into the
path of these traveling waves and you
can see the waves are reflected upward
for example if I move the reflector in
the other position I can reflect them
downward I can reflect them downward so
that they bounce off of the base of the
mirror and back up again and if I set
this reflector down you will see that
the waves also Bend around this barrier
they Bend around and actually go into
the shadow of the barrier and this is
what we call edge diffraction