Young's double-slit experiment. The experiment that proved that light was a wave. So
back in the 17th century this guy I don't know if you heard of him
Isaac Newton, he thought that light was made of particles ,which he called
corpuscles, which is actually just a fancy word for particles. I don't know why he didn't just
call them particles but you know i'm sure he had his reasons,
It was Newton! So Newton published his paper on the corpuscular theory of light and a
lot of the other physicists were kinda like no Newton that doesn't explain
diffraction. No Newton, that doesn't explain polarization. No Newton, what
the heck is a corpuscle? The theory was obviously pretty flawed but nobody had
any real evidence to prove Newton wrong and because Newton was such a big name at the
time his theory stuck around for over a hundred years, until this other pretty
cool guy Thomas Young came along with his famous double slit experiment that
went something like this. So we have two slits you know double slit. Now we're
going to place the light source directly behind the two slits so that when we
turn it on the light will shine directly through the slits so now we've got our
light source two slits and some distance away like a far distance away we have a
detection plane and this is usually just a screen or something something just to
show where the light is actually hitting now we'll turn on the light source and
shine the light throughout two slits so if like what particle-like what you doin
was saying we should see these two really bright spots on the detection
plane right directly in line with the slits if you need a real world
translation of this imagine that you have a table with a hole in it and
you're pouring sand all around this whole there's going to be a huge amount
of stand directly under the whole right
nothing fancy is happening the pot
chuckles of sand is falling directly under the whole or directly in line with
its well because it doesn't really make a mound where the most like is he is
going to show as the brightest spot of the detection plane or have the highest
intensity so if light were a particle / corpuscle we should see these two bright
spots of high-intensity on the detection plane right well that's know what
happened what young soul on the detection plane instead was this this is
the fault of reply was away you
oh let me break it down now if we imagine light as a wave when we turn on
the light source will get these waves emanating from the source so you're
probably used to seeing waves as squiggly sine waves right and this is
convenient because this tells us the amplitude but the drawback is that
there's only one dimensional for this diagram to make sense we need to see the
waves in 2d because we need to see the whole way front if you don't know what
away front is or you don't really know what I mean I've drawn a little picture
so hopefully this helps as you can see my artistic skill isn't quite developed
enough for 3d drawings but the idea is to show you what wave fronts are and
that the lines represent the peaks and the trough of the ways so the full lines
of the peaks and the dotted lines of the troughs so we have waves emanating from
the sauce now when the wavefront come into contact with the slits because of
wave diffraction the waves diffract around the slits forming cylindrical
waves now these cylindrical waves continue moving forward until they reach
the detection plane so now we have two sets of cylindrical weights and because
they came from the same source they have the same amplitude same frequency same
wavelength
what
it seems like we've got two identical monochromatic light waves overlapping in
and out of phase that's right guys who got both constructive and destructive
interference happening here
remember how the full lines are the peaks of the wave and the dotted lines
are the troughs of the way so where the pics are overlapping with the pics and
the traps are overlapping with the trucks we get constructive interference
where the peaks of overlapping with the Trump's we get destructive interference
now something a bit strange happens we get these kind of perfect straight parts
of interference
let's take the Center path for an example if we locate all the places
where constructive interference is occurring we get this kind of perfect
straight lines of constructive interference in other words we get these
perfect lines of high-intensity because the amplitude is getting bigger
we're going to have a higher intensity and this forms a bright spot on the
detection plane now if we look at the overlaps directly next to this path
we're going to see a perfect straight line of destructive interference because
of the wave amplitudes canceling out the intensity goes to 0 which leaves a dark
spot on the detection plane so if we find all these pods of interference and
follow them to the detection plane will see a bright spot here and here and here
and a dark spot here here here and here
this is what's called an interference pattern and they are totally
characteristic of waves if you ever seen interference pattern you know the waves
are involved i guess i just told you that in the beginning but where would be
the fun in that
so you may have guessed that they called interference patterns because they're a
result of wave interference constructive interference causes the bride spots or
sometimes called bright fringes and destructive interference causes the dark
spots or dark fringes you may have noticed that the middle bright spot is
brighter than the two
on the side I'm gonna leave this one up to you guys to figure out let me know in
the comments below if you think about it
so there we have it undeniable evidence that light is a wave so you might be
wondering why don't we see this effect in everyday life
I mean surely there must be a pair of slip like objects lying around somewhere
during daylight right i mean what about sunlight coming into my two bedroom
windows
well there are a couple conditions
the first is that the slips needs to be around the same size as the wavelength
of light for the light to the front to put this into perspective red light has
a wavelength of around 700 nanometers that's over a hundred times smaller than
the diameter of an average human hit we're talking about some pretty tiny
slits the second is that the light needs to be monochromatic this just means
light of all the same frequency or wavelength you know mano one chroma
color one color is all the same frequency
you know like red light has all the same frequency blue light is all the same
frequency
anyway sunlight is not 10 chromatic it's made up is all the colors of the rainbow
literally and that's why we don't see interference patterns everywhere in
everyday life the splits needs to be really really really small and the light
needs to be monochromatic so the moral of this story is you know about from the
whole like being wasting if you're ever feeling like you know he didn't do so
well on that test or sometimes you just don't get it
just remember that even geniuses like Isaac Newton get around sometimes
hey I'm room all the time maybe I'm a genius
uh-huh
hi guys season