Kind: captions
Language: en
[Applause]
previously on veritasium we saw how our
understanding of light has changed over
the centuries in the late 1600s hyans
proposed that light was a type of wave
while Newton considered it a stream of
particles this debate appeared to be
settled in 1801 by Young's double slit
experiment which showed light passing
through two slits produced patterns like
water waves but by 1900 it was clear
that light energy was not evenly
distributed as expected for a wave
rather on the smallest scales it comes
in lumps called Quant or photons so the
question is how does this affect the
double slit experiment here I have a
more contained double slit experiment
where there is a laser which fires a
beam through a single slit then through
a double slit and onto a screen where
you can see a well-defined interference
pattern there's a series of bright and
dark bands which are much easier to
observe than when I use sunlight because
with only one wavelength there are no
other colors involved this is what a
graph of intensity versus position would
look like for the interference pattern
what creates that pattern P consider the
bright spot in the middle the light from
each slit has to travel the same
distance to reach that point and hence
both waves arrive in Phase that means
crests with crests and troughs with
troughs so they add together and create
an interference maximum a bright spot
but if you look slightly to the left
there's a dark spot now that's because
light from one slit has to travel on an
angle and it has to travel an extra half
a wavelength compared to the light from
the other slit which means when this
light is arriving as a Crest the light
from the other slit is arriving as a
trough and so they cancel each other out
but if you go further left you see
another bright spot because now the
light from one of the slits has to
travel a full extra wavelength compared
to the light from the other slit and so
again they arrive in Phase crests with
crests and troughs with troughs creating
constructive interference and so we see
a bright spot of light and that's how
the whole pattern is created but what if
I decreased the intensity so much that
there wasn't a whole wave of light going
through there there were only single
photons then how could they interfere
with each other because there's only one
in the device at any one time so would
we still see an interference pattern
that is what we're going to find out in
order to make this work I had to line up
a very faint source and to see where the
light was going I had to shroud my head
in that black cloth but I finally have
the apparatus ready you can see up here
I have a frequency counter which
actually counts the number of photons
received per second at the detector the
detector is a photo multiplier tube
which is capable of detecting single
photons it's like a very sensitive eye
like the Frog's eye now right now
there's no light passing through to the
detector but there is still a bit of a
background reading and that's because I
can't block all the light out of there
plus there are cosmic rays passing
through this room which will also set
off the detector I'm plotting a graph of
the number of photons counted as a
function of position across the detector
if you have a look after 1 second the
distribution seems random there doesn't
seem to be any pattern in the
arrangement of those photons as they hit
the detector so maybe it's true a single
Photon can't interfere with itself
because it's just a localized point it
has to go through one slit or the other
but just to be sure let's add up the
results over a period of time and see if
any pattern
[Music]
emerges look at that you can clearly see
the same interference pattern that we
got when we were sending tons of photons
through but we're getting it out of
single photons we're counting up
individual photons and that pattern is
emerging as we aggregate the results
over time but how could this be
happening How could a single Photon pass
through both
slits well if we try to interpret these
results in terms of the objects we
experience every day they don't make any
sense a photon is something different to
a a macroscopic object it's not a wave
and it's not a particle it's a quantum
mechanical object and sometimes it seems
like it has properties of a wave and
sometimes it seems like it has
properties of a particle but ultimately
it is something totally different to
anything we've experienced before and
that's what makes this seem so
counterintuitive so what is light wave
or
particle the true answer I think is
neither though if you want you could
call it a wave AC
[Music]
is created by the ripples from the two
sources interacting with each other
where they meet up peaks with Peaks and
troughs with troughs the amplitude of
the wave is increased that's what we
call constructive interference but if
the Peak from one wave meets up with the
at the end of the last video I asked why
is it that the interference pattern is
made by blobs rather than thin lines of
light as it was in this experiment and
in the comments I saw two common answers
one was that the blobs were images of
the Sun the other answer was that it was
due to Heisenberg's uncertainty
principle now those were very thin slits
so it's true Heisenberg's uncertainty
principle would be involved and that
light passing through each slit would
spread out but the reason we actually
saw blobs was because they were images
of the Sun so if the sunlight comes
through one slit it defract out and with
spread out onto the bottom of the box
and light from the other slit would do
exactly the same and where those two
spreads overlap is where we saw the
three main blobs so there's a defraction
maximum created by one slit and another
defraction maximum created by the other
slit and those two
overlap but due to the light being in
different phases it cancels out in
certain positions creating those
separate images of the Sun uh