Kind: captions
Language: en
a few months ago I made a video about
schlieren imaging now that's a technique
used to visualize tiny differences in
air either temperature pressure
composition so you can see things like
the heat that comes off when you light a
match now in that video I asked you for
some ideas for what you'd like to see in
this leerin setup and I will be showing
you some of those in this video but I
also saw that there was a lot of
confusion about how exactly the setup
works and so I want to clear up that
confusion before we get going so one
thing is this mirror I said is a
parabolic mirror but then I said you can
think of it as a part of a sphere and so
people rightly called me out and said
well if it's parabolic it can't be part
of a sphere while that is true it's such
a small part of a sphere or a parabola
that the two shapes are pretty similar
on this scale so you could say it
approximates a sphere and you would not
be as far off as say in physics when we
say this cow approximates a sphere now a
parabolic concave mirror is a pretty
good approximation for a spherical
concave mirror now the second major
question is how do you get enough light
to make this work I mean some of those
shots I was showing you were shot at
2,000 frames per second and using only
one tiny LED as the light source how
could that even work
well I had similar kind of concerns when
I was going to set this up and so I
sought out the most powerful flashlights
that I could find I figured this would
be my light source so that light bounces
off the mirror now the focal length of
my mirror is 1.8 meters which means that
the light converges at the center of
curvature which is two times the focal
length three point six meters back and
you can see that here the reflected
light that comes off the mirror is
converging on this card that I'm holding
up you can see as we get closer and
closer right there look at how sharply
we have an image of the flashlight it
passes and forms a spot back here this
is essentially an image of the mirror
and look how bright it is we have
daylight outside okay
it's very bright but you can still see
this spot which comes completely from
the flashlight now let me try to set up
something in front of the mirror that
will produce a bit of a schlieren
pattern and let's check how that looks
you can actually see the schlieren
effect and you might notice this is not
a fantastic clarín image and that's
because of course this is not a point
source of light so my first thought was
cover it with tin foil and make a hole
on the front to reduce the size of the
light source now that has decreased the
brightness of this image but it's still
pretty bright and you can actually see a
really nice flaring effect that's taking
place already right here so if you
imagine sending all of that light down
the lens of a camera it's actually a
pretty bright image not a dim one but if
this works and produces the schlieren
effect we're looking for then why do you
need the razor blade well here's the
thing the differences in refractive
index that we're talking about are very
very tiny so the way the light is
deflected is just by the tiniest of
degrees so that light actually will end
up right here in this focal point with
all the rest of the light because all of
the light from the mirror is getting
sent back to this spot but some of it
the light that's been deflected will be
just ever so slightly off and if you use
this razor blade to knock off about half
of that bright spot you will be cutting
off more of that light which has been
deflected and by cutting it off you will
increase the contrast in the image now
as some of you pointed out an
alternative to using the razor blade to
cut the light is using colored filters
so here I got two different colors of
cellophane and if I position that so
that the focal point is right in the
middle of the divide then some of the
light will pass through this magenta
side and some will go through the
science side have a look what happens
when I put this transparent helium
balloon in front of the mirror now the
helium just deflects the light a tiny
amount but it's enough so that instead
of going through the magenta side it
goes through the science side and that's
why the balloon looks a different color
but
happens if I pop it do you see that
after the bloom pops the helium kind of
stays together and rises in the shape of
a balloon another thing you suggested
was lighting a barbecue lighter with a
match so I'm just going to release some
gas I'm going to let the flame travel up
the gas
[Music]
and the schlieren you can see the flame
traveling out and through that gas up to
light the lighter yeah and try blowing a
bubble with rainbow schlieren
another crazy idea of yours was to light
a ping-pong ball on fire because
ping-pong balls the good ones anyway
that they use in actual tournaments are
quite flammable so I'm gonna see how
that looks whoa it's caught fire
look at it go oh that is awesome
now a lot of your recommendations
involve sound for example trying to see
a clap
I tried this again and again and I tried
looking for a shock wave but it was
pretty hard to see even at 2,000 frames
per second and that's because with the
speed of sound being over 300 meters per
second even at that speed you would only
catch maybe one or two frames that would
include the shock wave in them and I
found it almost impossible to see so
looking at all these claps you can see
the air getting pushed out from between
my fingers but that's not traveling at
the speed of sound so it's not the
actual sound of the clap so I'm gonna
have to call in some backup if we're
going to do some schlieren
with sound so thank you for all of your
suggestions if you have any others
please leave them in the comments below
though it may take me a little while to
get back to you because next week I'm
flying to Australia and on the plane I
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you