Kind: captions
Language: en
[Applause]
[Music]
Where is the darkest part of a shadow? I
mean, the obvious answer seems to be
right in the middle. If you look closely
at a shadow, as you move the object away
from the wall, you'll notice that the
shadow gets a bit fuzzy. So, clearly the
edges are lighter. Now, we know that
light bends around corners. That's a
phenomenon called defraction. So, is it
defraction that's responsible for the
fuzzy edges of shadows? Actually, no.
The reason that shadows have fuzzy edges
is because most light sources like the
sun are not perfectly point objects. So
light from one edge of the sun is coming
in at a very slightly different angle
from light from the other edge of the
sun. And that's what gives us these
fuzzy edges. It's not defraction. But
defraction can play a significant role
in shadows. And in fact, it was the
cause of a massive debate about 200
years ago. In 1818, the French Academy
sponsored a competition to try to find
the best explanation of defraction. And
Augustine Fresnel entered this
competition with the suggestion that
light is a wave phenomenon. And just
like any ordinary wave, it bends as it
passes around an obstacle. But one of
the judges was Simeon Pon, a harsh
critic of the wave theory. He much
preferred Newton's idea that light was a
stream of particles. And to show just
how ridiculous the wave theory was, he
showed that Fresnel's theory would
predict a bright spot in the shadow,
right in the middle of the shadow behind
a circular object, that there would be a
spot almost as bright as if the object
wasn't there at all. And he thought this
was absurd. But my question is, does it
exist? Is the brightest part of a shadow
really in the middle? To find out, we're
going to have to do the experiment. So,
first I had to find some circular
obstacles. I selected spheres instead. I
got some marbles and some small
spherical magnets. Then, for a powerful
light source and a large distance to the
screen, I selected a lecture theater
with a projector. Now, the first thing
we're going to need is a small aperture
for the light to pass through. Uh, so
I'm going to poke a hole in this card
with a screw. It's important to have a
small aperture because you want the
light to be in phase. And that should
work if it's all coming out of this
small little aperture. But when I put a
sphere in front of it. All right. Hit
the lights.
Here we go.
I can't see a bright spot. I can't
really see anything.
So next I tried a cell phone flashlight.
Putting a marble in front of that. And
again, I could see nothing.
I don't see anything in the middle.
The idea with pusan spot is that light
should defract around a circular object
or a sphere. And because the center of
the shadow is equidistant from all the
edges of that obstacle, all of the light
should constructively interfere at that
point creating that bright spot. So next
I tried an overhead projector. I tried a
string of the small spherical magnets.
It's weird. Like I feel like I can see
it. Do you think you can see a bright
spot in the middle of the bottom one?
No.
I feel like in the very center of each
one, I can see a bright spot.
Oh,
I don't know. My eyes are just totally
going nuts up here, like staring at
shadows and trying to see what we want.
But this one is opaque and it's nice and
spherical. Set it down.
And if I adjust the focus,
you can see there is a bright spot right
in the middle of that marble.
So, the spot looks pretty good, but
something about it didn't sit right with
me. And you know, when you think
something is true, you really should try
as hard as you can to disprove it.
What if you used your fingers to kind of
make the edges less
spherical? Yeah. So, you know, that's
that I think is concerning. The fact
that we can still see a bit of spot even
though
it looks like you're
right. How is the light getting through
and getting there? What was really
happening was light was coming up from
the base of this overhead projector,
bouncing off this lens, back onto the
top of the marble, and then back up
through this lens and onto the wall. So,
finally, I decided to use a laser. I
resisted doing this earlier because in
1818, they didn't have lasers. In fact,
Pson didn't do the experiment. He didn't
think that you needed to. The idea of a
bright spot in the middle of a shadow
was just so ridiculous. And Fresnel
didn't do the experiment either, but one
of the other judges by the name of Argo
decided to actually do the experiment.
And when he did it, he saw something
similar to what I saw. So here I was
using the laser through a diverging
lens. And then that beam was shone onto
a marble resting on top of a loop of
tape. So you can see on the wall
basically exactly what you'd expect,
just a shadow of the marble. But when
you turn the lights off, there it is. A
bright spot right in the middle of that
shadow.
So the brightest part of a shadow is in
the middle as long as it's the shadow of
a circular or spherical object. Now this
spot is sometimes called Argo's spot
because he did the experiment and found
it. Sometimes it's called Fresnel's
bright spot because it was his theory.
But pretty frequently it's referred to
as Pson's spot, a harsh reminder that
it's not only your great achievements,
but also your greatest mistakes that can
be named after you.
We don't see Pan Spot in our day-to-day
lives for many reasons. One of which is
that most objects are not perfect
circles. Plus, if they have any surface
roughness really at all, that will
completely wash out puss spot. And
finally, most light sources are not
coherent. That is, the waves are not
coming all in phase, peaks with peaks
and troughs with troughs. So, you might
think you could never see puss spot
under ordinary conditions. But you can.
What you need to do is look at a diffuse
source of bright light like a
fluorescent tube or the blue sky. You
should see some small light specks
drifting around your visual field. Now,
those are caused by floaters, little
particles actually drifting around
inside your eyeball. And they can be all
sorts of different shapes, but some of
them are spheres. And so, they cast a
shadow on the back of your retina. And
right in the middle of that shadow is
Pan's bright spot. And that is what
demonstrated that light really is a wave
phenomenon. And you don't even have to
take my word for it. You can see it with
your very own eyes.