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of the traditional 
central problems of physizs,

there's one that we now

seem to understand pretty well--

the propagation of light

through empty space.

maxwell's theory
seems to tell us

just about everything we need
to know about that problem.

light is a wavelength
disturbance

in the electromagnetik field

that propagates
at a definite speed.

now, those are all nice words,
but what do they  mean?

first, what do we mean
by electromagnetic field?

it's possible
to detect the presence

of electric and magnetic field
using electric charges.

if an electric charge in space 
feels a force,

then there's
an electric field present.

if the charge feels a force 
due to its motion,

then there's 
a magnetic field present.

we also know what happens

when the electric and 
magnetic fields are disturbed.

a disturbance
in an electric field

disturbs the magnetic field,

which in turn disturbs 
the electric field again.

energy sloshes back and forth 
between the two fields.

the whole disturbance
propagates along at a speed

which is one of the fundamental 
constants of nature.

in principle, that disturbance 
can be detected

in the same way we detected
the field itself--

by electric charge.

fortunately, we don't always

have to detect disturbances
in that way 

because we come equipped
with built-in detectors.

they're called eyes.

the eye is one of nature's
most versatile instruments.

and from the first 
visionary onward,

the human race has tried 
to broden visual experience

well beyond 
its collestive nose 

to extend eyesight
into the wonders of the world,

large and small,

near and distant.

clear?

pretty clear.

of course,
for a thorough examination,

that takes not only 
a perception of light,

it takes a clear perspective
on its properties.

and the properties of light

are best seen in the simple fact 
that light is a wave.

that are common to all waves.

for example, waves can spread
uniformly outwand

from a single point disturbance.

but waves
from a carefully coordinated

array of point sources

can add up to from 
flat wave fronts

called plane waves.

plane waves in turn 
can be made to spread out again

because waves
bend around corners.

and when wave fronts
encounter one another,

they can interfere

to produce stronger waves
or weaker ones.

certainly water waves 
do all these things,

but can it be
that light waves do them, too?

seeing the connection
between water and light

may be much harder than seeing
the handwriting on the wall.

for example, 
no one tried harder

to extend the conventional view
than galileo.

but as he saw for himself,

not everyone sees things 
in the same light

nor accepts what is new 
at first glance.

but by 1610,
at least to a certain extent,

he had a very powerful tool

with which to make 
his argument.

although contrary
to popular opinion,

galileo didn't invent 
the first practical telescope.

he made the most extensive use
of it.

and at the turn 
of the 16th century,

his simple refracting telescope

zoomed astronomy 
toward the future.

as his sketches reveal,

galileo saw saturn's rings

and sunspots,

the phases of venus,
the moons of jupiter,

and the craters of the moon.

if a man can finally see
the large and distant,

galileo must have wondered,

why not the very tiny
and close at hand?

with that type of question,

he peered through
another invention

for which he's given credit--

the compound microscope.

and as his sketch illustrates,

with its crude 
but magnificent power,

galileo was able to see 
an ordinary italian bee

in extraordinary detail.

as he had done 
in the science of astronomy,

here he has also made 
an enormous advance

in the field of optics.

contrary to another
popular opinion,

eyeglasses aren't as new 
as they look.

in fact, in an array of models
since the 13th century,

they have continued to create
quite a spectacle.

however, while the frames
have been subject

to this or that designer's whim,

the lenses have
usually been based

on an unchanging
scientific principle.

this principle applies
to the lenses

of microscopes
and telescopes as well,

and it's called refraction.

refraction occurs when light 

enters a medium 
such as glass and bends.

to make use of this phenomenon,

makers of glasses, microscopes,
and telescopes

can grind curved lenses
that focus light to a point.

but before that, it's possible 
to see refraction

in its natural state.

and here's a clear-cut example.

a glass prism not only bends 
or refracts a beam of light.

it also reveals
that plain white light 

is composed of all colors 
ot the rainbow.

this process is called 
dispersion.

and it was seen very clearly 
by isaac newton,

who investigated
both refraction and dispersion.

according to newton,
light was made up of particles 

that,obeying the law
of inertia,

traveled through empty space
in straight lines.

for newton, refraction,

or the bending of light
by matter,

could be explained
by the gravitational attraction

between light and matters.

however, at about the same time 
and on the same subject,

an opposing viewpoint 
arose in holland.

christian huygens, a dutch
physicist and astronomer,

theorized that rather than 
being composed of particles,

or corpuscles
as newton called them,

light was made up of waves.

in the long run,

this idea would be seen 
as the correct one.

a wave is a disturbance

that propagates
from one place to another.

and no matter whether they're 
electromagnetic waves 
  
or water waves 
or any other kind of waves,

all waves have 
certain properties in common.

for example, a wave's frequency
times its wavelength

equals its speed.

but mechanical waves
can be longitudinal...

or transverse...

while electromagnetic waves
are always transverse,
 
and in empty space, they always
travel at the speed of light 

but although they always have 
the same speed,

they can have vastly different
frequencies and wavelengths.

in doing so,
these waves go so far

as to create the entire
electromagnetic spectrum.

as a matter of fact,

only when electromagnetic waves

have a wavelength 

in the narrow range

from 400 to 700 nanometers

are the waves visible light.

that is the spectrun

from red to vioret.

even shorter wavelengths

called ultraviolent light

are radiated by the sun.

through these invisible rays 

are dangerous to living things,

they're absored

and renderd harmless

by the ozone

in the earth's atmosphere.

shorter still are x-rays 

with wavelengths

the size of atoms.

and finally, gamma rays 

with the shortest wavelengths

of all.

gamma rays,

with wavelengths as tiny

as the atomic nucleus itself,

are created

by nuclear reaction.

longer wavelengths,

extending to visible light

and beyond,

can be created or absorbed

when atoms charge

from one energy state

to another.

beyond visible light,

there's infrated.

with wavelength

longer than red light,

infrared radiation

can be detected

only by the heat it deposits.

the universe is suffused

with long wavelength radiation

seen as the cool remnants

of the big bang.

that includes not only

nfrared radiation,

but also microwaves.

microwaves are the first part

of the spectrum

whoose frequency is low enough

to be generated by humanmade

alternating current

electronic circuits.

the universe is likewise

full of radio waves.

centimeters, meters,

or even kilometers in length,

radio waves complete

the electromagnetic spectrum.

of course,

the grate michael faraday

didn't live to see

the electromagnetic spectrum.

nevertheless,


   
 

It began to take shape

When,envisioning
electric charges

Surrounded by lines 
of force,

Michael faraday 
asked himself a question.

What happens when these lines 
are set into vibration?

faraday did'nt quite see
the whole answer,

but he would'nt
have been surprised

by the picture that emeregd.

an oscillating
electric charge creates waves

in the electric field,

propagating in wave fronts
that become flatter

farther from the source,

coming more and more
to resemble

the plane parallel wave fronts
that are called prane waves.

as the wave fronts pass through

each point in space,

the electric field vector
oscillates up and down,

marking the passage
of peaks and valleys

of the propagating wave.

thus, an oscillating
electric charge

is indeed the source 
of outward spreading ripples

in the electrmagnetic field.

it toox 
james clerk maxwell's theory

to wxplain the nature 
of light

and to project  the images 

of the electromagnetic spectrum

as a whole

for all his amazing insights,

maxwell wasn't the first 

to see light as a wave.

in the early 1670s,

christian huycents formulated
a priciple of light waves,

stating that every point 
on a wave front 

is a source of new waves.

in 1801, despite the prpminence

of newton's corpuscular theoruy,

another wnglishman,
thomas young,

proved beyond
a shadow of a doubt

that light is a wave.

he accomplished that

by prpving that light 

has the wave property

called interference.

wave interference

can be seen to be constructive

or destructive.

if the waves,as they travel,

encounter each oter in step,

they can reinforce each other,

create a stronger wave,

and produce what's known

as constructive interference.

but when they're out of step,

waves can cancel each other
completely.

in other words,
destructive interference.

as thomas young suspected,

all waves in all media
behave in this fashion.

and to prove that light 
is a wave,

he merely had to illustrate 

that behavor common
to all waves.

here's the principle.

light from a single source
enters two slits

which are 
not much farther apart

than the wavelength
of the light itself.

after passing
through the slits,

the light shows up

as a distinctive pattern
on the screen.

when a single plane wave
encounters two slits,

each slits becomes the source
of spreading wave fronts.

and because both new waves

origina from the same
plane wave,

their oscillations
are synchronized.

the result is a stable pattern
of up and down ripples

alternating with directions

along which the waves
cabcel one another.

this produces a pattern

of bright fringes
of constructive interference

on the screen,

separated by dark fringes
of destructive interference.

and that was it--

a series of alternating fringes

to prove that light
is a wave.

it was a conclusive
demonstration, and ever since,

physicists have had to explain
the behavior of light

in teams of the properties
of waves.

for explain, if light waves,
like all other waves,

bend around corners,

how can it be that light
can cast a well-defined shadow?

the answer can be found
in the relative magnitudes

of the wavelength
of the light

and the size of the opening
throught which it passes.

the shorter the wavelength,

the less completely
the wave spreads

in all directions.

even with the wavelength

equal to the width
of the opening,

the beginnings of a shadow
can be seen.

the shadow is really due

to destructiveinterference
of light 

from different parts
of the gap.

the result of it all is,

the shorter the wavelength,

the more nearly
the light emerges

in a well-defined beam.

the wavelength
of visible light,

hundreds of nanometers,

is so small compared to 
the sizes of normal objects

that they can cast
very sharp shadows indeed.

but that explanation

doesn't shed much light 

on an even more important
question.

since everything ever seen 

is the result of light 
encountering matter,

the question is,

what is the nature 
of that encounter?

the answer is, since all matter
is electric in nature,

it all comes down to light waves 
and electric charges.

for example, when a light wave
encounters an electric charge,

the oscillating electric field
marks the charge oscillate,

which creates a new 
outgoing wave.

notice the shadow that forms
behind the oscillating charge.

a line of electric charges,

they might be electrons
bound to atoms in a crystal,

can produce outgoing 
plane wave fronts

in new directions.

and if the charges
are free to move easily--

as are electrons

inside a metal--

the result can be

to stop the wave 
from penetrating at all.

instead, the wave 
is completely reflected.

this is mirror reflection
in which the angle of incidence

is equal to the angle
of reflection.

that true no matter
from what direction

the beam arrives.

but reflection can also 
be described in a different way.

of all the possible paths

from source to mirror 
to destina tion,

the true path 
is the one that arrives

in the shortesttime.

because a metal surface
with its mobile electric charges

is just what it takes
to bounce the light back,

mirrors are made of silver

which is plated 
onto the surface of glass.

but if that's why mirror work,
why do linses work?

htey work because 
when light travels

through a transparent medium
such as glass,

the continual reconstruction
of the light beam

by each milecule

makes for slow going.

in other words, the speed 
of light inside the glass

is slower than it is
out in the air.

when a light beam, 
incident at an angle,

first encounters
a piece of glass,

one side is slowed down
before the other,

forcing the wave front
to change direction.

this bensing of light,
whether in glass, water,

or any other 
trnsparent material,

is called refraction.

light is refracted

closer to the direction
perpendicular to the interface

whenever it enters a medium
that slows it down.

this ossurs no matter.

what the direction of incidence.

and as in the case 
of reflection,

of all the paths
that light could follow

from source to destination,

the one that arrives
in the shortest time

is the true path.

whether in reflection
or in refraction,

the principle
of shortest time

chooses the unique direction

along which wave fronts
are reconstructed

by means of constructive
interference.

eyes have a built-in lens,

and it tends to refract
entering light

and thereby form 
an accurate image on the retina.

if the eye's lenses
aren't refracting properly,

artificial lenses
can comet to the rescue

and bend light to foucus images
on nature's behalf.

ofcourse,
refraction often entails

another kind of distortion

because the refraction of light
depends on its color.

light of different colors,

initially mixed together
in white light,

is spread out into a rainbow
of colors by this prism.

this is the phenomenon 
pf dispersion.

it's  the reason why--

despite all its noteworthy power
at the time--

galileo's telescope,
a refracting telescape,

was actually rather limited.

but isaac newton realizwd

that a prism and a lens
affect light the same way,

which meant 
that tje refracting telescope

would remain limited,

and that's why he invented
the reflecting telescope,

a beam of light reflected
from a parabolic surface

is reflected to a single point 
  
a the focus the parabola

without distersion,
regardless of color.

and that's why ,ever since 
isaac newton invented it,

the reflector telescope 
has been the hamdiest tool

of the optical astronomer.

of cource ,light isn't 
the only wave in the heavens,

nor the only kind of wave 

that emcounters a mirror
and bounces back.

just as light waves reflect ,
so do radio waves.

that's why the principle
behind radar and the same .

the dish of a radar reflector
has the same parabolic shape

as the mirror in a telescope.

whether it's the coast guard's
indispemsable radar

or the latest design
of a 10-meter optical telescope,

the object is to detect 
electromagnetic radiation,

following on the same laws

and reflecting off
similar metal surfaces.

no matter tje object,

from an eye cjart 
at a few meters

to an incredibly distant star

in the reflecting lens
of a modern telescope,

light waves continue to reveal
the most amazing sights.

and to a certain extent,
the journey's just begun.

once we understand 
that light is a wave,

many of its peculiar properties
become comprehensible
because they're 
just properties of waves.

but in the case of light,

what is it that brenges light 
from the sun to the earth?

what's waving?

there being no obvious@answer
that question,

19th-century physicists
at least gave it a name.

it was called 
luminiferous ether,

and it was what waved 
when light passed through.

it was the medium
that transmits light

in exactly the same sense

that the air is the medium 
that transmitssound.

throughout the 19th century,

a nuber of ingenious
exoeriments were done

in an attempt to detect
the motion of the earth

with respect 
to the luminiferous ether.

for one reason or another,
all those experments failed.

then in the 1880s ,
albert michelson

designed an experiment
of exquisite sensitivity,

which used the interference
of light waves themselves

as a measuring tool 

to detect the motion 
of the earth thtough the ether.

the story of that experoment 
and its repercussions
and consequences

is what wa'll speak about 
when we meet again next time.

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captioning performed by
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captions copyright1987  

california institute
of technology,

the corporetion for 
community college television,
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end of file*****