Today, I'm going to talk about light. Light is an electromagnetic phenomenon
and already in the sixteenth century, way before Maxwell, a lot of studies were done
of the interaction of light with water and with glass. And the kind of experiments that were done
as follows, say this is air, I call that medium one, and this is water, call that medium two
and I have a light beam that strikes this surface. Light comes in like so and I define this angle
as the angle of incidence and I call that theta one. This is the normal to the surface
and we call that the angle of incidence. I will see now that some of that light
is reflected, reflected with an "L",
as in lion, and some of that light goes into the water,
and we call that refracted, refracted, with an "R", as in Richard
and this angle, we'll call theta two. And it was a Dutchman, Willebrord Snellius,
who, in the seventeenth century, found three rules that govern the relation
between these three light beams. The first one is that this beam, this beam
and this beam are in one plane. As you see,
that is my plane of the blackboard. The second thing that he found,
that this angle, theta three, which is called the angle of reflection,
is the same as the angle of incidence. That was known before him, of course. And then the third one, which is the most
surprising one, which is called after him, which is called Snell's Law,
although his name was Snellius, is that the sine of theta one
divided by the sine of theta two, if we go from air to water,
then that ratio is about one point three. If you go from air to glass,
it's a little higher, it's like one point five or so. He introduced the idea of index of refraction,
which I will call n, as in Nancy, index of refraction. For vacuum, the index of refraction,
per definition, is one, but it's very closely the same in air,
we always treat it as one in air. And in water, the index of refraction
is approximately one point three and in glass, depending upon
what kind of glass you have, it's about one point five. And so we can now amend this law, Snell's Law,
by writing here n two divided by n one and one being the index of refraction of the
medium where you are, your incident beam, that's why I put a one here and two
being the index of refraction of the medium where you're traveling to. You're refracted into this medium. And so you see, indeed, that since water
is one point three, and air is one, that this ratio for air to water
is one point three. And this is called Snell's Law. And it is immediately obvious that if you
go from air to water, or you go from air to glass, that angle theta two
is always smaller than the angle theta one, because this number is larger than one. But if you go from water to air,
then the situation is reversed and that's what I want to address now,
that's actually quite interesting. So now, my medium one is now water
and my medium two is now air. And so now, I go from here to here and so here I have my angle of incidence
theta one and here I have my angle of reflection,
that is the theta three and now here, I have my angle theta two. And so if I write down, now, Snell's Law,
then I get the sine of theta one divided by the sine of theta two
is now n two divided by n one, but n two is one,
divided by one point three, if we go from water to air. And what is so special here is that-- theta two can obviously never
be larger than ninety degrees. And so if you substitute in here,
theta two is ninety degrees, then you will find that theta one, then,
is about fifty degrees. And if you apply this equation
and you substitute for theta one, an angle larger than fifty degrees, you're going to find the sine of
theta two being larger than one, which is nonsense. It cannot happen. And so nature ignores Snell's Law
and nature says, "Sorry, I can't do it," and what nature now does,
if the angle of theta one is too large, in this case, with water, larger than fifty degrees, this is not there anymore, and all the light is now being reflected off
that surface. And we call that total reflection. Total reflection. So total reflection happens when theta one
is larger than a certain critical angle and the sine of that critical angle,
for which I write cr, is n two divided by n one, but there is a condition. And the condition is that n one
must be larger than n two. If that's not the case,
then there is not total reflection. And total reflection
is actually very interesting, it has practical applications,
which I will discuss with you shortly, but I first want to do a demonstration in which I want to show this to you. I have here, water
and here is air and I have a laser beam
which I can shine in and I can change this angle theta one
and slowly increase it and you will see that
when I approach fifty degrees, first of all, you will see that theta two
increases, increases, increases, and then, when I approach the critical angle
and exceed it, then we have hundred percent reflection. Let me first turn on the laser
so that there's a little bit of light, and I'm going to show it to you there
for those of you who are not sitting very close and that means I have to set
the light situation-- OK, so there you see, the light coming in,
just the way we had it on the blackboard,
this is the way it comes in, in water, this is the reflected part and this is the one
that is refracted into the air. So that is this one here. And now I'm going to increase that angle, when I touch the table,
the water will start to wiggle a little and you will probably see that. So I'm going to in-- OK, I decre--
I'm going to increase that angle, look how I'm increasing it. Look, that theta two is getting larger,
is going to approach the ninety degrees, I increase theta one, I increase theta one,
look at theta two, almost ninety degrees, I'm very close to the critical angle now,
I'm almost at it right now and now all the light is being
reflected. Hundred percent reflection. A remarkable phenomenon. And this has practical applications. And we're going to show you some of these
practical applications, too. The most important practical application
is fiber optics. If I have a fiber and it is properly designed,
so this is a fiber, and some light comes in here
and it hits here, so this is, say, some plastic,
or glass and this is air, if this angle of incidence is larger
than the critical angle, hundred percent reflected. And so nothing comes out in the air,
hundred percent reflected. Here, again, the critical angle is exceeded
and so hundred percent is reflected and you can go through this whole thing for miles
on end. You can put even knots in there,
as long as you never exceed the critical angle, that light will propagate and there will never
be any loss of light and that's why people are very much
interested in this. You can transport even images,
as I will show you, through fiber optics. I have here, uh, fiber optics,
which has four thousand fibers in it, fifty microns in diameter each and we have a laser beam here
and the laser light will come out, I will show you the laser light shortly there
and it doesn't matter what I do with the fibers, I can even go hundred eighty degrees
and shine them there, as long as, inside the fiber,
I always exceed the critical angle. Uh, oh, I don't want the television any more,
so we can turn this off and let's here have the laser light. There it is. Here you see the laser light. OK, now look at this,
this bundle that I have here. I turn it into an absurd snake,
almost like an S. All the light still comes out. So it goes al the way through, I'm going to turn it
hundred eighty degrees around, turn it to the wall there. There it is. So there's an amazing phenomenon
that this light doesn't get out in the air, it stays inside the fiber and that's the idea
behind fiber optics. I have another, uh, application of fiber optics
right here which is very similar. You can send an image through fiber optics. This is my fiber optics, now,
thousands of small fibers. And I send a message in here, this side,
an image, could be a person, could be a text. And here we have a TV camera. And we can watch that image on this side of
the fiber appears that image, and this television camera will be able
to see that image. And let's see whether we can
show you that message. OK, I have to do something here again,
I think you're going to-- ah, there it is! So you actually can see the individual fibers,
you see them? How-- how interesting. Each one,
these are individual fibers. And their diameter is probably not much more
than fifty or a hundred microns. Let's see what message there is for you. Oh man, oh man,
exam three. You don't want to hear about that,
do you? Well, that depends
on what the message says. Problem... no... [laughter] no... Problem one! Problem one. The... figure... oh, oh, oh, oh, below...
oh, I must have... that's the wrong message, I couldn't possibly meant to give you away the
exam, of course, so I apologize for that, that I showed you the wrong message. But at least it demonstrated to you that you
can send an image through fiber optics, even secret messages. Newton had an interesting explanation
for Snell's Law. Newton was the man of planets. He was the man of particles, masses, accelerations, F equals ma. And so his explanation came with particles. He says light,
light are particles. So if this is the surface between, let's say,
air and water, Newton argued as follows. If light comes in, it has a certain speed,
v one. And therefore,
it has a horizontal component. And it has a certain
vertical component. And he says if this light
ends up in water, at the moment that it
reaches the surface, it gets an acceleration perpendicular
to the surface. Why? He didn't tell us. But he said, it gets an acceleration
perpendicular to the surface. In other words, this horizontal component
does not change. That remains what it was. But this component changes,
this becomes substantially larger, depending upon the index of refraction,
of course. And so the new velocity
is now in this direction. And so you see that, indeed, the angle theta
one is larger than the angle theta two. But Snell's Law immediately follows from this. The sine of theta one--
this angle, is theta one. So this angle, is theta one. So that's this velocity
divided by this vector. And the sine of theta two is this velocity
divided by this vector. But these two velocities are the same. And so you immediately find that this ratio
is v two divided by v one, a great victory for Mr. Newton. Except there was one problem, maybe,
that it means that v two is larger than v one, and so Newton argued that the speed of light
in water is larger than the speed of light in air. And in glass, of course,
it would be even larger. Now, there was a Dutchman and the name of Dutchman
was Christian Huygens. H-u-y-g-e-n-s. And this gentleman suggested that perhaps
light are not particles, that they are waves. And this guy came up with a genius idea,
which we now know as Huygens' Principle, at least you call it Huygens' Principle,
but we don't call it Huygens' Principle. You see, this sound, "u y",
is pronounced in Dutch as "ouwe". None of you can say "ouwe"
unless you're Dutch. To make it worse, this sound you don't have
either in English. That's a "kkkkhhhh". None of you can say "kkkkhhhh"
unless you're Dutch. Let alone that you can say the combination,
Huygens. Anyone who comes forward to me
after this lecture and who knows how to pronounce the word "Huygens", has to be Dutch. I will be kind to you, and call it, today,
Huygens' Principle. [laughter] So Huygens came with the following idea. Here is a source of electromagnetic waves. And these electromagnetic waves
propagate out in a spherical way. Not unreasonable. And so you even see here,
the wave crests. And so he defined the surface
at the leading part of the wave, where all points are in phase,
he called that the wave front. So this is the wave front. And he now postulated that each point
of the wave front individually oscillates at the same frequency
as the source, and produces spherical waves. We call them secondary waves,
often also called wavelets. And that the envelope of the wave front
of the secondary wave is now the new wave front. So it works as follows. Each point that you can choose,
you can choose as many as you want to, starts to oscillate at the same frequency
as the source and produces, on its own,
spherical waves, there they go, and the new wave front is then here. And this is called Huygens' Principle. Of course, he doesn't give any explanation
of why these points do that. He hypothesized that. And this principle can explain Snell's Law
in a very easy way. I want you to read up on that in your book. And you will see that it is very easy to explain
Snell's Law with this principle, except that now, you will conclude that the sine of theta one divided by the sine of theta two is v one divided by v two
and therefore, Huygens predicted
that the speed of light is lower than the speed of light in air,
whereas Newton predicted that the speed of light in water would be higher
than the speed of light in air. So the question now was,
who was right? Are lights particles,
or are they waves? Well, the wave-particle idea of light
has been a very long-standing issue in physics, and I will show you, I think, next week,
or maybe after the exam, how Young in 1801, conclusively demonstrated
that light are waves. So it looked like Huygens
was going to be the winner. On the other hand, I showed you last lecture
that light can behave like particles. Photons, bullets, tomatoes,
radiation pressure, that's particles. the whole thing,
the whole ball of wax. And so maybe Newton was right,
maybe they are particles. Well, they're both right. There are times that you can actually interpret
what you see best be assuming they're waves, and that there are times that it's much better
to assume that they are particles, like in the case of the radiation pressure. But of course, the key question now is,
who was right in terms of the speed of light? Is the light going faster in water,
then Newton was right, or is the light going slower in water,
then Huygens was right. Needless to say
that the Dutchman was right. The speed of light in water is lower
than the speed of light in air. When we arrived, the speed of light in vacuum,
we used Maxwell's equations. And they allowed us to conclude that the speed
of light, much to everyone's surprise, depends on epsilon zero and mu zero
in a very simple way, I will call it for now, v, that was one over the square root
of epsilon zero, mu zero. And we call that c. If you had used Maxwell's equations
as they are valid in materials, in dielectrics, and also in materials
that have magnetic properties, then it would be exactly
the same derivation, but you would have seen a kappa here,
the dielectric constant and you would have seen here
the magnetic permeability. Kappa, if you're not in air, but in glass and water,
is always larger than one. So you see in front of you
that the speed of light in water is lower than the speed of light in air. This can also be written as c divided by
the square root of kappa divided by kappa M and we would nowadays, simply write that
as c divided by n. So the index of refraction is really the square root
of the product of the dielectric constant and the magnetic permeability. Now, kappa and kappa of M are very strong
functions of frequency. And that's not so surprising,
because at very high frequency, the intrinsic electric
and magnetic dipoles, which are being aligned
by the alternative external fields, cannot follow quickly enough. A field wants to drive them in this direction
and it wants to drive them back and forward and back
and there's just not enough time to do that. And so you expect that at high frequencies, the values for kappa are lower
than at low frequencies, which is exactly what you see. In the case of kappa M, that's only important
when you deal with ferromagnetic materials, because with paramagnetic and diamagnetic
materials, kappa M is always one anyhow. Or very close to one. I have chosen water as an example, to show
you the dependence of kappa on the frequency. This is on the Web, so you can download it
and make yourself a copy. And so if you look here, this is for water--
then, you see there the-- at low frequencies, at zero Hertz
and even at radio frequencies, at hundred megaHertz--
this is hundred megaHertz, this is, uh, radio--
they are are radio waves-- notice that the dielectric constant in water
is about eighty and, um, at, um, at visible light--
these are the frequencies of visible light-- it's way lower. We just discussed that. The oscillations go to fast,
the electric dipoles can't follow it. And so the index of refraction then,
for radio waves, at hundred megaHertz,
is roughly nine, and so the speed of those waves in water
is nine times lower than the speed of light in air-- we call it speed of light, but it's the speed,
of course, of the radio waves then-- and in the case of visible light,
you see that visible light in water, the speed is only one point three times lower
than what it would be in air, or, of course, in vacuum. The frequency effect
is very noticeable. If you take red light and blue light,
they have different frequencies and therefore, the index of refraction
is different for red light and blue light. If I take water, the numbers I'm going to give you
are for water, then the index of refraction for red light in water
is one point three three one-- but the index of refraction for blue light in water
is one point three four three. And we're going to use these numbers,
shortly, to get a deeper understanding of the formation
of the rainbow that's behind this,
of course. And so you notice that the blue light
is one percent slower in water than the red light. And this phenomenon
that you see there, that the speed of electromagnetic radiation
depends on the wavelength, depends on the frequency,
we call that dispersion. It is a good thing that sound in air
is not dispersive, because -- just imagine that high frequencies
would travel faster than low frequencies, just as an example. Or, or slower, for that matter. It would mean, then,
that if you go to a concert and you would listen to the violins
and the bass, that the violins would reach you first, and then the sound of the bass would reach
you later and the farther you are away from the orchestra,
the worse that would be. If the effect were very strong,
here in 26-100, someone sitting in the back row
could not even understand my words, because the high frequencies would reach
that person at a different time than the low frequencies. So sound in air is non-dispersive. But glass and water are dispersive for light,
and it's very noticeable. If I take a piece of glass and I give it this shape,
the shape of a prism and I shine some light on here,
light from these light bulbs, or light from the sun,
for that matter, then I can apply Snell's Law here. I know the angle of incidence,
theta one. I know the index of refraction, this is for water,
but you can look them up for glass, of course and then you will see there is a difference
of the index of refraction for red light that there is
for blue light. And so, when you reach this side of the prism,
again, you have to apply Snell's Law and when you do that, you will see that the red
light doesn't come out at the same angle that the blue light come out,
but the two diverge. That is the result of the fact that
the indices of refraction are different, but also the result of the fact that we have
this particular shape, funny shape, namely that this side of the glass
is not parallel to this side. And so if you put a screen here,
you will see colors, you can make a spectrum, you can convince yourself that the light
from these light bulbs is just not white light, but that it contains many, many colors. Well, it has to contain many colors
because if I look at that gentleman there, he's wearing a red shirt. Where do you think that red color
is coming from? It must come from the light bulb,
so there must be red light in there. The woman setting next to him
is wearing a green shirt, so there must also be green light in this--
this light. And the same is true for sunlight. But the beauty is that,
making use of the dispersion, you can decompose the white light
into the individual colors and make a spectrum. If you take a piece of plane-parallel glass,
which is window glass, then you're not going to see colors,
because, if now I shine light on here, white light from the light bulb or from the sun,
it is true that it will obviously refract in here. But when you apply Snell's Law again, here,
then it will come out-- all the colors will come out
in the same direction. So the red light comes out here
and the blue light comes out in the same direction. And your brains are very special. If your brains see all colors coming
from one direction, they say, "I see white light." Look at the light bulb. You say, "Ah, that's white light." But look at this gentleman, you say, "Hey,
it's red, it must come from that light bulb." So your brains are special
in the sense that they think that the combination of many colors
is white. And I can show that to you in a--
in a rather convincing way. You see here a disk. And I would assume that you see colors
on that disk. If not, you have a problem. And I can fool your brains. What I can do, I can rotate that disk so fast
that your brains get so mixed up that they're going to say to you, "Yes,
that's white light." So let me first give you
some ideal light on that disk, and I'm going to spin it up a little. So you agree with me, right? You still see colors, yes? Still see colors, right? OK. You still see colors, right? Yeah, hahaa. Haha. Hahahaha. This is white as it can be for me. Not too surprising, right? The same situation
when I look at the light bulb. All these colors are processed by my brains
in such a way that they think, or they-- well, they actually make you see
white light. It's as real as you can have it. And so this raises the subject
of the illusion of colors, which is what I want to discuss with you
for the remaining part of this lecture. If you ask a physicist, "When
do you see certain colors? When to we see red?
When do we see blue or green?" Then chances are, that he would
give you a standard answer and he would say, "Well, that depends on, uh,
on the wavelength in air. "If you tell me what wavelength you're on,
then I will tell you what colors you will see." And I have here a transparency
which makes the connection between wavelength and these colors. It's also on the web,
so you can also download this. And so if the wavelength of light in air
is about in this range, one angstrom is ten to the minus ten meters,
yes, you will probably say that's red light. When the wavelengths get shorter, in this range,
you would say, "Yes, that's green light." And when the wavelengths get even shorter,
you would say, "Yes, that's blue light." And you can't see any wavelength shorter
than this, you're getting to the ultraviolet here and you can't see any wavelength
longer than this, that's infrared and our eyes are not sensitive
for those wavelengths. So this is all nice and dandy,
but we still have the problem of the-- the fact that if you mix all the colors together,
like the rotating disk, and like looking at the light bulb, that our brains still tell us
that we are seeing white light. So maybe matters are not really as simple
as we think. And this scheme about colors
has been worked out in quite some detail already in the seventeenth
and in the eighteenth century, when it was discovered that there was
such a thing as primary colors. Maxwell did some research on that,
and Helmholtz, even the poet Goethe
did work on this. They discovered that there are primary colors,
and when you mix light, we call this additive mixing, the three primary colors are green, violet, and red and when you mix paint, the three primary colors
are yellow, blue, and red. And the idea behind this is that if you mix the--
the three primary colors in the right proportion, that you can make many colors. I want to show you a color triangle,
which is the recipe, tells you how you have to mix these colors
and in order to do that, I'm going to make it dark,
but not all the way dark. So here you see the color triangle
and the color triangle has in the three colors-- the three corners, the colors red,
one primary color and then here it has violet and then up there it has green. And now, I'll tell you how this recipe
has to be used. So I'm going to draw here a color triangle
and we have red here and we have green there, and we have here, violet. And if you look at this color triangle,
you see all the colors that you can imagine. Sort of. You see yellow and you see here purple
and you see orange, you even see white. So how do we make these colors, now? Well, suppose I want to make
this color here. So that's this color, say. Then you draw three lines,
one, two, three, from the three corners and this is the amount of red
that you have to put in, and this is the amount of green
that you have to put in, in the light and this is the amount of violet light. And if you do that, in that ratio,
you would get the color which is here. If you want to make very nice yellow--
oh, let's see, let's make some very nice yellow, which is all the way here,
at the edge you don't need any violet, you can do that
exclusively with green and with red, so let's go to this point here,
which is yellow, it would mean then, that I have to put in this much red
and I have to put in this much green. And if I add more and more red,
I go along this line, then I end up here and I get orange. So I could make orange here
by simply increasing the amount of red-- oh, this should not be violet right?
Ooh-ooh, ooh-ooh, this is green. Oh, you should have yelled at me. So this is green and so
if I want to make orange here, then I simply have to give it
more red and less green. And if you go to an extreme
and you want to make white light and you go bingo,
right there in the middle, well, then, you have to give it this much red,
this much green and this much violet. And that's the idea that we just saw,
you mix all the colors and we mix up your brains as well, and your brains then say, "Gee,
ah, that is white light." So the idea behind
the three primary color theory is that our eye cells,
the cells that we have in the retina, respond differently
to the three primary colors and that color sensation, which is, of course,
what the brains are telling you, those are the messages that are being sent
to the brains, and they are processed here, that they are
the result of the mixing of these three responses. And the theory is quite successful. I'm going to try to make for you the color yellow
by mixing these three colors green and violet. And if I want to make yellow,
then we already argued, all I need is green
and I need red, I don't even need violet. And I'm going to do that with a--
with this nice little box here. I will raise the screen because we don't need
the screen any more and I don't think I need the, uh,
the slide any more, um, John. And this box-- this box has three lights in there. Red, green, violet. And I can change the intensities. Yes, I can show you, I can make the--
the red less strong and I can do the same with the green,
I can make it less strong. And I can do the same with the violet. And so if I want to make,
for instance, that yellow, then I can do that
with green and red alone, and I have to give it a lot of green
and a little bit of red. So a little bit of red
and a lot of green. Can you adjust it a little? Hmm, I see yellow, don't you? Who sees yellow? OK. So we make it a little orange,
then we have to give it a little bit more red, yeah, I give it a little bit more red,
oh, it becomes orange. See, so I'm marching, now, here,
give it a little bit more red and you make it orange. And what I can even do, I can give them all three
the maximum strength and I can turn it to white. So we will fool the brains again,
all these colors, like the rotating disk, like the light from the sun,
like the light from the lights here. I think I see white light. Your color TV is based on this principle. You have three electron guns
in your color TV. And there are three different chemicals
on the screen of your television. And these chemicals are in the form
of very small dots. And if electrons hit one of these dots,
they become violet and the other chemicals become green
and the other chemicals become red. And so the whole idea is, now,
that each beam hits its own chemical dot, that's the way they arrange things. And by mixing the various intensities, you can mix the intensities
with three primary colors and you can see all colors on television. It works very well. Of course, if you haven't adjusted
your television properly, you may have noticed that sometimes faces
are reddish, or faces are greenish. Well, that's a matter of just adjusting
those three guns appropriately and then you can be very successful
and it's very impressive. This, uh, three-color scheme
works quite nicely. So, in many cases, the three primary color
theory is quite satisfactory. But there are cases
where it fails bitterly. And those are the ones
that I want to discuss for the remaining ten minutes
and forty seconds. Already, in the nineteenth century, it was known that there were problems
with the three-color theory. Mr. Benham, in 1895, invented a top
which is named after him, it's called the Benham top. I have one in my office on my table. It is a top that has just black lines on it,
no colors. You rotate it
and you see colors. And we copied that top for you
and we have that here, this is a copy,
it's a large copy, the top is only this small
that I have. This is a Benham top. I hope you will agree with me
that this is black and white. Any one of you see colors,
let me know now. I will ask you to leave, then. [laughter] OK, so no one sees colors. Good. So now, I'm going
to rotate that for you, and you will be surprised,
what you're going to see. So there is the Benham top and I'm going to rotate it somewhere
in the ballpark of about seven Hertz, five to seven Hertz,
let's take a look. Black and white. "Hee poppelepee" [Dutch expression],
what am I seeing? I'm seeing colors. Not very bright, but I am seeing colors. I see some rusty brown,
right there in the middle and then I see something
of a grayish-green, maybe, and dark blue further out. Who sees colors? Who doesn't? Oh, you're color-blind, then,
that happens. Well, you see colors,
just with black and white rotating. And, to make it even worse,
I can reverse the disk. I first have to stop it,
otherwise we burn out the motor and I can reverse and remember,
the rusty brown was in the center. And now we're going to rotate it
the other way around. And look again what you see. You're now seeing the rusty brown
near the edge. You see the colors reversed. A lot of research, uh,
was done in this area. But a complete neurophysiological explanation
is still not available yet, even though there are several very successful models that can predict what our brains will see. And when your color cells are stimulated
with flicker light, there are phase delays between the incident-- incident light and the response, the messages that are sent to your brains, which are currents of course. And the phase delay is different
for different colors. So what we're going here,
we're fooling the brains, we're sending flicker light to the brains
with different phase delays, the phase delays in the center are different
from the phase delays further out. And so the brains, then,
process that in the usual way, and they say, "Well, I'm sorry,
but you're going to see colors." And that's what you're seeing. The most fabulous example
of where the three-color theory fails, or at least, is very incomplete, is the work done by Edwin Land
in the early fifties. Edwin Land, very famous man,
he was the inventor of the Polaroid film. He pioneered color theories and became
very famous for a particular demonstration that I will do for you here. He gave me two slides,
I got them personally from Edwin Land. And these two slides that I'm going to show you
are black and white slides. That is non-negotiable. I'm going to show them to you,
they are as black and white as this disk. He took one of those slides
by taking a photograph of something and what that something is
you're going to see very shortly. And the other black and white slide,
he also took, again, black and white film, but he put a red filter
in front of his camera. But believe me,
it is a black and white slide. So you're going to see black and white slides. And then I'm going to do something special
with those slides and therefore,
I'd rather go there now and then explain things to you
as they come along. And so I have to make it very dark
oh, the screen has to come down, of course, we're going to need the screen,
because we're going to see slides. So two black and white slides. So the first black and white slide
is this one. I hope we will all agree
that this is a black and white slide. And the second black and white slide
is of the same scene. This, by the way, was taken
through a red filter. But it is black and white. The next one was not taken through a red filter,
but, I hope we all agree, is black and white. If I put a red filter in front of this
black and white slide, you see exactly what you would have predicted, namely that, yeah, it's like looking
through a red piece of glass, the whole world
turns a little reddish. Very boring. Although some kids like that. So this is what you're going to see. I can do the same with the other one, this is the one that Edwin Land
photographed through a red filter. If I put a red filter in front of it,
you're going to see what you expect, reddish, pinkish colors. OK. What do you think you're going to see
if we project one black and white slide on top of the other
black and white slide? Well, let's face it. Let's be down-to-earth. Black and white plus black and white
will remain black and white and that's what you see now. One is now on top of the other. You may not notice that,
but I will take one away... and add it again. Black and white plus black and white
gives black and white. Now, I'm going to ask you to sit very firm
in your chairs, because you're going to fall
off your chairs if you're not careful. I'm now going to put in front of the slide
that Edwin Land took through a red filter, I'm going to put that red filter in front
of my projector, only through that slide. The other one remains as it was. And there we go. And now what do you see? You see colors. Is this a miracle? Well, maybe it is. I see yellow here, I see green here,
I see some dark blue. Who also sees colors? Just say yeah. [Chorus of "yeah"] Who doesn't? Good for you. Isn't this amazing? Two black and white slides. That's all you're seeing and then you see
this silly red filter, which normally would give you only
a little bit of pinkish, reddish light. But when you put one on top of the other,
something bizarre is happening in your brains. Your brains are so incredibly
mixed up that they really think
you're seeing yellow there. And they really make you think
that you're seeing green there. A reasonable question now, is to ask,
if we took a picture of this, you take your camera with color film,
what will you see? Will you see colors,
or will it be black and white? Yes, you will see colors, but the colors will be different from the way
that you and I perceive them right now. So now you can ask yourself the question,
well, "What are not the real colors? The ones that you and I see,
or the ones that our picture will record?" Well, I think that's a
meaningless question. There is no such thing as right or wrong
in these matters. Our brains are very complicated
and whatever they show us, that's the real thing for us. Reality is very relative. And if you're color-blind, which quite a few
people in my audience must be, just a matter of statistics, then they have
a different reality altogether. Reality is only in the mind
of the beholder and it all depends on how your brains
are processing messages. The message that I'm giving you
for this weekend is, have a good time, but by all means,
start working on your exam three, which certainly
is not an illusion. [applause]
