One of the fundamental properties of
waves, light
waves, or any other kind of wave, is
interference.
The idea that a wave, since it is an
oscillating positive
and negative signal, can interfere with
another wave, of the same wavelength.
So if the two positives of these two waves
superpose
on top of each other, then the result is
more intensity.
If the positive of one superposes on the
negative of the other, that's called
destructive interference.
And then you get a, a null.
Or a minimum of the intensity.
Ordinarily, this is a little bit difficult
to
observe in your everyday life with light
waves.
But, we have here, a carefully controlled
situation, where we can demonstrate that.
What we've built is an interferometer,
which
is diagrammed on the board, behind me,
here.
You can see that we're taking our green
laser beam And
we're splitting it into two approximately
equal pieces, using a beam splitter.
So half the light transmits through, and
half the light reflects off.
Then these two beams
travel to two different mirrors.
And then return back to the beam splitter,
and recombine at the beam splitter.
So now, that recombined beam is what you
see.
That spot on the wall over there.
So if I block one arm of the
interferometer, you see the other beam
over there.
And if I block the other one, you see the
first one.
Right, and you won't see any interference
if one or the other is blocked.
But if we superimpose the two of them,
what
we see is, at certain points within that
bright circle,
we see a dark stripe through the middle,
and that dark stripe
is due to the destructive interference of
one beam against the other beam.
Now, this destructive interference is
extraordinarily sensitive to the path
length that one beam has traveled compared
to the other one.
And, what that means is, that if we shift
one beam of
the interferometer, the length of one arm,
by just half a wave length.
Then, that goes from constructive
interference to destructive interference.
So, we'll shift
the interference pattern.
So, an interferometer is incredibly
sensitive to very small motions, because,
remember, the wavelength of light is only
a few hundred nanometers.
So if I move this mirror by just a 100
nanometers or so, it will lead
to a shift in the interference pattern,
and I can do that just by blowing on it.
Interferometers are incredibly
sensitive devices for measuring very, very
small changes in the positions of objects.