## Subtitles section Play video

• Welcome to another episode of

• Michael draws on pieces of white cardstock

• Meets...

• Michael's toys

• That's right, today we have a combo episode for you,

• and we're gonna be talking about...

• vision.

• Let's say you want to see something.

• Alright, let's say you want to see, um...a black line,

• uuuuh wonderful.

• Now, to see, you're going to need something that can receive photons,

• So how 'bout we put a retina right...here

• oooh, that's a beautiful retina.

• Now, we see because light either reflects off of an object,

• or is emitted by the object.

• And that light contains information about the object.

• But here's the problem:

• Let's take a look at a point on the object like this one,

• I'll call it point "A" for "bottom"

• Now, light is leaving point A in all directions;

• you can see it from, you know, anywhere.

• But here's the problem:

• some of that light might land on the retina right here,

• but light from another point,

• like, uh...this one

• I'll call this point "B" for "top",

• might also fall in the exact same spot on the retina.

• So you wind up with this big, blurry mess of light information that makes no sense.

• It's kind of like what you would see if you took the lens off of a camera.

• In order to see,

• in order to form an image,

• we need to build a one to one correspondence

• between points on the object

• and...where light from them lands on the retina.

• The way our eye does it

• (in an extremely simplified way)

• Is by using a pin-hole.

• So I'm now gonna block

• the light coming off of this object reaching our retina

• with a...

• *draws*

• opaque plane,

• right here,

• peeerfect.

• But this plane is gonna have a tiny hole in it,

• a pin hole.

• Now watch what happens.

• When this light flies off,

• some...

• in fact just one ray of light that is leaving "A",

• intersects with the pinhole.

• Only one line connects two points

• on this Euclidean plane.

• And it will intersect that point, our pinhole,

• at a particular angle,

• and it will come through on the other end...

• like this!

• So here on the retina,

• we have information about point A,

• the bottom of our black line.

• Pretty cool, pretty cool.

• And notice that because we're using a pin-hole,

• any light rays that are leaving B

• with a trajectory towards...

• ...

• this part of the retina, are getting blocked

• by this plane right here.

• Only light rays from B that intersect...

• with that pinhole get through.

• But the angle they intersect at will be unique,

• So!

• The place they land on the retina will also be unique.

• If we choose a point that's just a little bit above A,

• I'll call this one A prime (A'),

• this ray

• that goes through the pinhole

• will have a slightly different angle

• and will thus come out...

• slightly...differently...

• *mumbling* nnnsortoflikethiss

• andthenitsgonnacomeout

• there it is,

• and so A' will be about here.

• As you can see, by using a pinhole,

• we have created a one to one correspondence

• between points on the object we're looking at,

• and points on the retina.

• We are constructing an image,

• of this black line, AB,

• on the retina that happens to be upside down.

• This is really how your eye works;

• the light information that lands on your retina

• is an upside down version of whatever you're looking at.

• luckily we have brains, and our brains know to turn things right-side-up again.

• This pinhole way of seeing explains why things appear smaller when they're further away.

• Watch this.

• Let me draw the same object, this black line, AB, but I'm gonna draw it further away.

• I'm gonna draw it...

• I wanna make sure that it's about the same height.

• It doesn't have to be perfect because this is just a little illustration,

• but let's say that we have our object over here,

• there's its bottom, there's its top,

• now take a look at the paths of the light rays that pass through that pinhole.

• I'm gonna use a straight edge here just so I can get this right.

• and...let's see what color should I use?

• Uh, I like this orange.

• Alright, so light rays, that are reflecting off of point A,

• pass through the pinhole,

• and they come through onto the retina like this.

• Ah, wow,

• So now, when the object is further away,

• point A corresponds to a point on the retina

• that's below where it corresponded when the object was closer.

• Let's take a look at point B.

• mmmkay

• Light from B that has the correct trajectory to pass through the pin-hole

• will come out the other side and land on the retina right there.

• Well, my gosh!

• If A is one edge of the object and B is the other,

• look how much smaller...the black line's image on the retina is going to be

• than when it's close,

• and it is this big.

• From that A... down to that B.

• This is geometrically what's going on

• when an object is seen from further away.

• The image they put on our retina is literally smaller.

• But this isn't the only way you can create an image!

• Another way to do that

• is to grab another sheet of paper...

• yeaah, beautiful!

• *cough*

• and watch Michael draw on more pieces of white cardstock.

• Now let's say that we are going to look at a line,

• alright, here it is, and I'll even give it the same endpoints,

• A and B.

• But this time, what we're going to project onto the retina

• will not... be a one to one correspondence due to a pin-hole

• but will instead will be a one to one correspondence

• created by some sort of magical filter

• that only allows light rays to go through

• that strike the surface of this filter at a right angle.

• What I mean by that is that light flying off of point A,

• on a trajectory like this,

• OOoooh...

• That is not a right angle, nope!

• This light gets absorbed or reflected away, something like that.

• However, light leaving point A like this,

• awwww, yeaah 90 degrees!

• This light is able to pass through the object,

• come out the other side,

• and land on the retina.

• Each point on the object will correspond to just one point on the retina

• that is... at exactly 90 degrees.

• So if this is point A',

• only light like this will be able to pass through the filter

• and reach this side and give us A'.

• Same with B, there we go, and there's B.

• Notice that in this case, the image that we are forming is right side up.

• It's not flipped like it is when it went through the pin-hole.

• Uh, just to be very clear, if there's a ray leaving from A,

• that happens to have a trajectory like this,

• that would bring it exactly to B,

• in which case we don't have a one to one correspondence, we've got a mess,

• it doesn't matter because of cource this light ray won't go through,

• it's not hitting at a 90 degree angle,

• so we have no problem.

• But here's what's interesting! As you can see,

• the dimension of the black line AB,

• the actual object in the world and the image formed on the retina

• are the same size!

• How cool would the universe look if things did not shrink in apparent size as they moved away from us.

• It might be kind of scary, exactly

• but who knows what it would actually look like

• OH WAIT! There's a way to know.

• Thank you...

• *paper flops onto floor*

• thanks to minerals.

• I have here some fantastic samples of various minerals.

• This is a piece of ulexite.

• Ulexite is a borate mineral,

• that as you can see is made of fibers

• that all go in one direction, they're all parallel to one another.

• Now ulexite will often have kind of dark colored sort of brown...issues in it.

• The rest of these rocks are selenite, which is a variety of gypsum,

• And it also is made out of, as you can see,

• parallel fibers.

• Because this mineral only allows parallel light rays to travel through,

• there is a one to one correspondence between light information coming from a point

• on whatever the mineral is on top of

• and on the surface, the other side of the mineral.

• For this reason, looking through the mineral isn't like looking through something that's transparent,

• Instead, an actual image of what is below is created on top.

• Ooh yeah, look at that!

• Here's the selenite,

• and...

• there's the image.

• Anyway, why am I bringing these up?

• Well, if our eyes were not eye balls, but were instead

• loooong pieces of minerals,

• like ulexite or selenite,

• and we literally had to touch our eye organs to whatever we wanted to see,

• it wouldn't matter how far away the thing was, it would always be the same size.

• Take a look at this.

• This is an enormous piece of selenite, which is perfect because,

• when you look from this camera right here

• I'm pointing at this camera right above me

• when you look from there down at say this number 30,

• the light from that 30 is converging towards the lens on that camera or towards your eye.

• And so it's smaller if it's further away.

• But!

• oh wow this is like falling apart into sharp fragments...

• Be careful...

• I don't know actually how sharp they are.

• ...

• Hannah could you come lick this?

• It looks like...it looks like if you inhaled this stuff you'd be in a lot of trouble.

• I will...

• keep going though.

• Because your knowledge is more important than my health.

• without the selenite, the distance between the edges of the number 30,

• converge right away.

• But!

• With the mineral, they spend a whole lotta time travelling parallel to one another,

• and only after that do they begin converging.

• So it's as if the ruler is closer to you.

• That's why the number 30 looks bigger when the crystal is on top of it.

• Why is this coming apart so much?

• I wonder if I could eat it...

• *licks*

• Eeeah! You know what?

• The table ith thalty from like...

• having sweaty hands and sweaty Michael around it.

• *exhales*

• This episode is supposed to be about optics properties, not taste, but

• *sniffs*

• *licks*

• It's funny it really tastes, uh, cold,

• but of course it's just room temperature, it just has a much better...

• uh...uh,

• capacity to conduct heat, than air does.

• Cause I'm not tasting cold, I'm just losing heat

• from my tongue

• more quickly to this than I do to the air.

• Eauh now there's a bunch on my tongue.

• what ha

• *spits*

• What happened to that?

• Why is it...shedding?

• Anyway, that...um,