Kind: captions
Language: en
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what is light what is light light light
what is light that's a good question
isn't it what is
light isn't it an element um light is
brightness I guess we have auras we all
have auras which are light yes they are
it lights up the room it makes it not
dark what's the difference between blue
light and red light the color it goes in
your eyes and then you see stuff they
range from white to red to orange to
Green it's like the chakras of your body
can you see my aura uh no not
particularly right now is it too bright
out it's very sunny out here today does
that make it harder to see someone's
aura not necessarily if I was to explain
it to a blind person right it would be
it would be the difference uh you see
nothing whatsoever as a blind person
whereas I see things in front of me to
be fair the question of what light is is
not an easy one for centuries the
greatest Minds in science debated this
issue in the late 1600s Newton proposed
that light was a stream of particles or
cor pusles he proposed this in his
treates Optics but at the same time a
Dutch physicist named hyans proposed
that light was a wave and this debate
raged on until it was settled by the
experiment I've recreated today Thomas
Young's double slit experiment to make
sure I got the experiment right I went
to the the original source with the help
of Brady Heron I managed to get into the
Vault underneath the Royal Society in
London where there I found Thomas
Young's handwritten notes from 1803 I
brought into the Sunbeam a slip of a
card about 13th of an inch in bre and
observed its shadow either on the wall
or on other cards held at different
distances besides the fringes of colors
on each side of the Shadow the shadow
itself was divided by similar parallel
fringes smaller dimensions
wow this is an experiment so simple that
you could make it at home and yet so
fiddly that I have never seen it before
done with sunlight I was thinking about
doing it in a box like a like a fridge
box and you can take it out on the
street taking it out on the street could
I possibly interview you guys for about
a minute we're doing a science
experiment I have here is an empty box
and this is a little eyepiece where we
can look in and this is a hole and I'm
going to place this slide above that
hole and if you look closely you'll see
that there's two openings
very narrow opening side by side it's a
double slit now before we have a look we
need to tilt it towards the Sun a little
bit so we want the sun to hit this
double slit directly what are we going
to see on the bottom of the Box the
obvious thing you think you're going to
see is you're going to see two two lines
two lines on the bottom of the box two
bright bands two little lines yeah I
think it'll be one one line instead of
two I could expect to see the whole box
lit up that probably be a collider scope
of some sort a bunch of colors probably
yeah rainbow different colors there have
a look
you expected to see kind of one
line is that what you see no I see dots
how many it's one Circle oh there's one
there's one in the middle strongest two
either side the two on the outside are
multicolored and the one on the middle
it's just white it's kind of a rainbow
the rainbow of color as well quite a few
colors and lots of little dots like
there are more dots appearing I think I
can even see more dots spreading
along yeah that's amazing
yeah I can see tons of dots now not tons
but I can see dots spreading across that
way either side yeah definitely isn't
that amazing yeah that's
incredible and that's just nothing else
apart from two slits two
slits
come that's incredible but all we're
doing is we're putting a light through
two very narrow slits side by side so
how does this make any sense there's
some kind of principle involved in that
the average person is not familiar with
that's the only explanation I'm really
confused by it but I like to find out
why people were debating is light a wave
or is it made of particles so what
causes that well if light were behaving
as particles you would expect them to go
through each slit and just produce a
bright spot underneath so we would see
two bright spots on the bottom of the
box but if lights behaving as waves then
the wave from one slit can interact with
the waves from the other slit I've got
to demon demonstration here on a little
Pond where we can see this with water
waves I have two sources of ripples
which are basically like the two slits
when I create ripples with a single
Source they travel out with circular
wavefronts nothing particularly
surprising there but if I add a second
source of ripples then we start getting
an interesting
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pattern this pattern 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 trough from the
other then we get destructive
interference and there's basically no
wave there and this is exactly what was
happening with the light when the light
from one slit met up peaks with Peaks
and troughs with troughs they
constructively interfered and produced a
bright spot but if the trough from the
wave from one slit met up with the peak
of the wave from the other slit they
would destructively interfere and you
wouldn't see any light there it's light
canceling itself out this is basically
the same as like having two drops of
water fall in a swimming pool get
exactly the same they go and overlap as
this Ripple over overlaps with those
ripples down the bottom you get a series
of you get like a bright spot and then a
dark spot and then a bright spot then a
dark spot and a bright spot now there's
a slight complication which is that
sunlight is composed of many different
colors and they have different
wavelengths so obviously they're going
to meet up at slightly different points
and that's what caused the rainbowing
effects as we go further from the
central maximum you saw the ones to the
right were slightly colored because the
reds are going to meet up at different
places than the blues and that's all
that makes the color differences a
different wavelengths exactly that's
amazing so the difference between red
that red bin over there and the green
the green part is just I'm seeing that
just different W and that's how we bring
in all these beautiful colors all around
us
exactly that's amazing I I'm amazed oh
good on you thanks B thank you I have
been enlightened literally
now you may have noticed in that
experiment that the light on the bottom
of the box was not in the shape of slits
rather they were more kind of round
blobs and I want to know why that is can
you write an answer for me in the
comment section and I'll give you a hint
one of the videos I've linked kind of uh
is suggestive of the
answer so convincing were the results of
Young's doubl slit experiment that the
scientific Community concluded that
light must be a wave there is no way way
it could be a particle