Episode 40: Optics - The Mechanical Universe
Summary
TLDRThis script delves into the physics of light, explaining its wave-like nature and propagation through space. It discusses the electromagnetic field, the role of electric and magnetic fields, and how disturbances in these fields lead to light's propagation. The script explores the historical development of understanding light, from Galileo's telescope and microscope to Newton's work on refraction and dispersion. It also touches on the wave properties of light, such as interference and diffraction, and the technological advancements like the reflecting telescope. The concept of the luminiferous ether and the failed attempts to detect it are also mentioned, setting the stage for future discussions on the nature of light and its interaction with matter.
Takeaways
- 🌐 Maxwell's Theory provides a comprehensive understanding of how light propagates through empty space as a wave-like disturbance in the electromagnetic field.
- 🔍 The electromagnetic field can be detected by electric charges, with forces indicating the presence of electric and magnetic fields.
- 👀 Human eyes are natural instruments that detect disturbances in the electromagnetic field, allowing us to see.
- 🌊 Light exhibits wave properties such as spreading from a point, forming plane waves, and bending around corners, similar to water waves.
- 🌈 The behavior of light waves, including interference and diffraction, was demonstrated by Thomas Young, supporting the wave theory of light.
- 🔭 Galileo's use of the telescope and microscope expanded our ability to observe both macroscopic and microscopic worlds.
- 🌈 The phenomenon of refraction, explained by the bending of light as it passes through different media, is fundamental to the function of lenses in devices like glasses, microscopes, and telescopes.
- 🌈 Dispersion, the spreading of light into its component colors, occurs due to different colors of light refracting by different amounts.
- 🌌 The electromagnetic spectrum includes a wide range of wavelengths, from radio waves to gamma rays, each with distinct properties and applications.
- 🚀 The concept of the luminiferous ether, a medium for light waves, was proposed but never empirically confirmed, leading to significant debates and experiments in the 19th century.
Q & A
What is the electromagnetic field and how can its presence be detected?
-The electromagnetic field is a field that is present whenever electrical and magnetic forces are exerted on charged particles. Its presence can be detected using electric charges; an electric field is present if a stationary charge feels a force, and a magnetic field is present if a moving charge feels a force due to its motion.
How does a disturbance in the electromagnetic field propagate?
-A disturbance in the electromagnetic field propagates as a wave-like disturbance, with energy sloshing back and forth between the electric and magnetic fields. This disturbance travels at a constant speed, which is one of the fundamental constants of nature.
What is the role of the human eye in detecting disturbances in the electromagnetic field?
-The human eye acts as a natural detector of disturbances in the electromagnetic field, specifically in the form of visible light. It allows us to perceive light and thus 'see' the disturbances as part of our visual experience.
How did Galileo contribute to the advancement of our understanding of light and the cosmos?
-Galileo made significant contributions by using the telescope to observe astronomical phenomena such as Saturn's rings, sunspots, the phases of Venus, the moons of Jupiter, and the craters of the Moon. He also used the compound microscope to see tiny objects in detail, advancing the field of optics.
What is refraction and how does it relate to the lenses used in glasses, microscopes, and telescopes?
-Refraction is the bending of light as it passes from one medium to another, such as from air into glass. It is the principle used in the design of lenses for glasses, microscopes, and telescopes to focus light to a point by grinding curved lenses that take advantage of this bending effect.
How did Isaac Newton's experiments with a prism contribute to our understanding of light?
-Isaac Newton's experiments with a prism revealed that white light is composed of all the colors of the rainbow, a phenomenon known as dispersion. This showed that light could be separated into its constituent colors, and it also led Newton to propose that light was made up of particles.
What is the difference between longitudinal and transverse waves, and which type are electromagnetic waves?
-Longitudinal waves involve the oscillation of particles in the direction of the wave's travel, while transverse waves involve oscillations perpendicular to the direction of travel. Electromagnetic waves are always transverse, with the electric and magnetic fields oscillating perpendicular to the direction of wave propagation.
How does the wavelength of light affect its behavior, such as casting shadows?
-The wavelength of light affects how it bends around corners and how sharply it can cast shadows. Shorter wavelengths do not spread out as much, allowing for more defined shadows. Visible light, with wavelengths in the hundreds of nanometers, is small enough to cast very sharp shadows compared to the sizes of normal objects.
What is the principle of shortest time, and how does it relate to the reflection and refraction of light?
-The principle of shortest time states that light takes the path that allows it to reach a destination in the least amount of time. This principle explains why light changes direction upon reflection or refraction, as it chooses the path that ensures the fastest reconstruction of the wavefront through constructive interference.
How does the phenomenon of dispersion affect the performance of telescopes?
-Dispersion causes different colors of light to refract by different amounts, which can lead to chromatic aberration and a spreading of light into a spectrum of colors. This limits the performance of refracting telescopes, which is why reflecting telescopes, which do not suffer from dispersion, were developed.
What was the luminiferous ether, and why was it significant in the 19th century?
-The luminiferous ether was a hypothetical medium through which light was thought to propagate in the 19th century. It was considered the medium that transmitted light, similar to how air transmits sound. Experiments to detect the motion of the Earth through the ether, such as the Michelson-Morley experiment, were significant in the development of our understanding of the nature of light and led to the theory of relativity.
Outlines
🌌 Understanding Light Propagation
This paragraph introduces the fundamental understanding of light propagation through empty space, as explained by Maxwell's Theory. It discusses how light is a wave-like disturbance in the electromagnetic field and how disturbances in the electric field can affect the magnetic field, and vice versa, creating a wave that propagates at a constant speed. The human eye is highlighted as a natural instrument for detecting these disturbances, and the paragraph sets the stage for exploring the properties of light, such as its wave nature and the ability to form plane waves, which are foundational to understanding light's behavior.
🔭 Galileo's Contributions to Optics
The second paragraph delves into Galileo's significant role in advancing the field of optics. Despite not inventing the telescope, Galileo made extensive use of it to observe astronomical phenomena like Saturn's rings and the moons of Jupiter. The paragraph also touches on the invention of the compound microscope, which allowed Galileo to see minute details of insects. The discussion then shifts to the historical use of eyeglasses and the scientific principle of refraction, which is crucial for the function of lenses in both microscopes and telescopes. The concept of dispersion is introduced through the example of a prism, which separates white light into its constituent colors, a phenomenon investigated by Isaac Newton.
🌈 The Electromagnetic Spectrum
This paragraph explores the electromagnetic spectrum, explaining that electromagnetic waves are transverse and travel at the speed of light in a vacuum. It details the vast range of frequencies and wavelengths that make up the spectrum, from gamma rays with the shortest wavelengths to radio waves with the longest. The paragraph also discusses the role of the atmosphere in protecting living things from harmful ultraviolet light and how different wavelengths interact with matter, leading to the creation of the entire spectrum. The historical contributions of Michael Faraday and James Clerk Maxwell are acknowledged for their work in understanding the nature of light and the electromagnetic field.
🌈 Light as a Wave: Interference and Shadow Casting
The fourth paragraph focuses on the wave nature of light and Thomas Young's demonstration of light's interference, which confirmed its wave-like properties. It explains how constructive and destructive interference create patterns of bright and dark fringes, respectively. The discussion then moves to the question of how light, being a wave, can cast sharp shadows, which is attributed to the relative sizes of the light's wavelength and the objects it encounters. The paragraph also describes how light waves interact with electric charges, leading to the creation of new waves and the phenomenon of reflection, which is fundamental to both mirrors and lenses.
🔄 Reflection and Refraction of Light
This paragraph discusses the principles of light reflection and refraction. It explains mirror reflection, where the angle of incidence equals the angle of reflection, and how this principle applies to both mirrors and lenses. The concept of refraction is introduced, detailing how light slows down when it enters a medium like glass, causing it to change direction. The paragraph also addresses the issue of dispersion, which causes different colors of light to refract at different angles, leading to the spread of white light into a spectrum of colors. The historical shift from refracting to reflecting telescopes is mentioned, highlighting Isaac Newton's invention of the reflecting telescope to overcome chromatic aberration.
🌐 The Lumiferous Ether and the Nature of Light
The final paragraph ponders the question of what medium light travels through in empty space, a concept historically referred to as the luminiferous ether. It discusses the 19th-century experiments, including Michelson's, aimed at detecting the Earth's motion through this hypothetical medium. The paragraph sets the stage for further exploration of the nature of light and the experiments that would challenge the ether theory, hinting at the upcoming discussion on the consequences of these experiments for our understanding of light and the universe.
Mindmap
Keywords
💡Electromagnetic field
💡Wave-like disturbance
💡Maxwell's Theory
💡Refraction
💡Dispersion
💡Interference
💡Electromagnetic spectrum
💡Luminiferous Ether
💡Reflection
💡Transverse waves
Highlights
Maxwell's Theory provides a comprehensive understanding of the propagation of light through empty space.
Light is a wave-like disturbance in the electromagnetic field that propagates at a definite speed.
The presence of electric and magnetic fields can be detected using electric charges.
Disturbances in electric and magnetic fields propagate energy back and forth between the two fields.
The human eye is a natural instrument for detecting disturbances in the electromagnetic field.
Galileo's extensive use of the telescope revolutionized astronomy by observing celestial bodies in detail.
Galileo's compound microscope allowed for the detailed observation of small objects.
Refraction is the bending of light as it enters a medium like glass, which is used in lenses of glasses, microscopes, and telescopes.
A glass prism demonstrates that white light is composed of all colors of the rainbow through a process called dispersion.
Isaac Newton's investigations into refraction and dispersion led to the understanding that light is made up of particles.
Christian Huygens proposed that light is made up of waves, a theory that would later be proven correct.
All waves, including electromagnetic waves, share common properties such as frequency and wavelength.
The electromagnetic spectrum includes a range of wavelengths from gamma rays to radio waves.
Visible light is a narrow range of the electromagnetic spectrum with wavelengths from 400 to 700 nanometers.
Thomas Young demonstrated that light exhibits wave properties, specifically interference, through the double-slit experiment.
The nature of light's interaction with matter is determined by the electrical properties of the matter.
Mirror reflection and refraction are explained by the principle that light takes the path of least time.
The reflecting telescope, invented by Isaac Newton, overcomes the limitations of dispersion in refracting telescopes.
The principle behind radar and telescopes is the reflection of electromagnetic waves, including light and radio waves.
The luminiferous ether was a hypothesized medium for the transmission of light, which was later disproven.
Albert Michelson's experiment aimed to detect the Earth's motion through the luminiferous ether using light wave interference.
Transcripts
foreign media
[Music]
[Applause]
thank you
thank you
I'm the traditional Central problems of
physics there's one that we now seem to
understand pretty well
that's 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 wave-like disturbance in the
electromagnetic field that propagates at
a definite speed
now those are all nice words but what do
they mean
first of all what do we mean by the
electromagnetic field
well it's possible to detect the
presence of electric and magnetic field
using electric charges if there's an
electric charge in space and it feels a
force then there's an electric field
present
if the charge is in motion and it 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
an electric field disturbs the magnetic
field which in turn disturbs the
electric field again energy sloshes back
and forth between the two fields and 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 that we
detected the field Itself by means of an
electric charge
fortunately we don't always have to
detect disturbances in that way because
we come equipped with built-in
detectives of disturbances in the
electromagnetic field 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 broaden
visual experience well beyond its
Collective nose
to extend eyesight into the wonders of
the world large and small near and
distant
clear
be clear
what they say less 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 means light must have properties
that are common to all waves
for example
waves can spread uniformly outward from
a single point disturbance
but waves from a carefully coordinated
array of Point sources can add up to
form flat wave fronts called plane waves
plane waves in turn can be made to
spread out again because weaves bend
around corners
[Music]
and when wave fronts encounter one
another they can interfere to produce
stronger waves or weaker ones
certainly water waves do all these
things
can it be that light waves do them too
[Music]
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 that which 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
and with the question of that nature he
peered through another invention for
which he's given credit
[Music]
it was 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
[Music]
has he done in the science of astronomy
here he had also made an enormous
advance in the field of Optics
[Music]
contrary to another popular opinion
eyeglasses aren't as new as they often
look
in fact in an array of models since the
13th century they have continued to
create quite a spectacle
[Music]
however while the frames have been
subject to this or that designers win
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 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 of the rainbow
this process is called
aspersion
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
[Music]
fraction or the bending of light by
matter could be explained by the
gravitational attraction between light
and matter
however at about the same time and on
the same subject
an opposing Viewpoint arose in Holland
[Music]
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
and in the long run his idea would be
seen as the correct one
[Music]
a wave is a disturbance that propagates
from one place to another
[Music]
and no matter whether they're
electromagnetic waves
or water waves or any other kind of
waves all waves have certain properties
in common
[Music]
for example
a waves frequency times its wavelength
equals its speed
but mechanical waves can be longitudinal
or transverse
[Music]
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
and 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 4 to 700
nanometers are the waves visible light
that is the Spectrum from red to Violet
even shorter wavelengths called
ultraviolet light are radiated by the
Sun
though these invisible rays are
dangerous to living things they're
absorbed and rendered harmless by the
ozone in the Earth's atmosphere
[Music]
shorter still are X-rays with
wavelengths the size of atoms
[Music]
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 reactions
[Music]
longer wavelengths extending to visible
light and Beyond can be created or
absorbed when atoms change from one
energy state to another
Beyond visible light there's infrared
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 started it all
that includes not only infrared
radiation but also microwaves
microwaves are the first part of the
spectrum whose frequency is low enough
to be generated by human-made
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 great 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 didn't quite see the whole
answer
but he wouldn't have been surprised by
the picture that emerged
[Music]
an oscillating electric charge creates
waves
that propagate along the lines of force
at the speed of light
these ripples are transverse waves in
the electric field
propagating in wave fronts that become
flatter and flatter farther from the
source
coming more and more to resemble
the plain parallel wavefronts that are
called plane waves
[Music]
as the wave fronts pass through each
point in Space the electric field Vector
oscillates up and down
[Music]
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 electromagnetic field
[Music]
it took James Clark Maxwell's Theory to
explain the nature of light
and to project the image of the
electromagnetic spectrum as a whole
but for all his amazing insights Maxwell
wasn't the first to see light as a wave
[Music]
in the early 1670s Christian Huygens
formulated a principle of light waves
stating that every point on a wavefront
is a source of New Waves
and in 1801 despite the prominence of
Newton's rival corpuscular Theory
another Englishman Thomas Young proved
beyond a shadow of a doubt that light is
a wave
he accomplished that by proving that
light has the wave property called
interference
[Music]
wave interference can be seen to be
constructive
or destructive
if the waves as they travel encounter
each other in Step they can reinforce
each other create a stronger wave and
produce what's known as constructive
interference
[Music]
but when they're out of Step waves can
cancel each other completely
in other words destructive interference
[Music]
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 light
exhibits interference
the behavior common to all waves
here's the principle
light from a single Source enters two
slits which are side by side and 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
[Music]
when a single plane wave encounters two
slits
each slit becomes the source of
spreading wave fronts
and because both New Waves originate
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 cancel one another
[Music]
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 terms
of the properties of waves
for example 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
through which it passes
the shorter the wavelength the less
completely the wave spreads in all
directions
foreign
even with the wavelength equal to the
width of the opening the beginnings of a
shadow can be seen
foreign
is really due to destructive
interference of light from different
parts of the gap
and the result of it all is
the shorter the wavelength the more
nearly the light emerges in a
well-defined beam
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
and the answer is
since all matter is electrical 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 makes 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
[Music]
and if the charges are free to move
easily as our electrons inside a metal
the result can be to stop the wave from
penetrating at all
Instead The Wave is completely reflected
[Music]
this is mirror reflection
in which the angle of incidence is equal
to the angle of reflection
that's true no matter from what
direction the beam arrives
[Music]
but reflection can also be described in
a different way
of all the possible paths from source to
mirror to destination
true path is the one that arrives in the
shortest time
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 mirrors work
why do lenses work
they work because when light travels
through a transparent medium such as
glass the continual reconstruction of
the Light Beam by each molecule of the
material 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
spending of light weathering glass water
or any other transparent material is
called refraction
[Music]
foreign
[Music]
closer to the direction perpendicular to
the interface
whenever it enters a medium that slows
it down
this occurs no matter what the direction
of incidence
[Music]
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
[Music]
whether in reflection are in refraction
principle of shortest time chooses the
unique Direction along which wavefronts
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
does that look pretty clear if the eyes
lenses aren't refracting properly
artificial lenses can come to the rescue
and bend light to focus images on
Nature's behalf
foreign
[Music]
of course 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 of dispersion
the reason why despite all its
noteworthy power at the time
Galileo's telescope a refracting
telescope was actually rather Limited
[Music]
but Isaac Newton realized that a prism
and a lens effect light the same way
which meant that the refracting
telescope would remain limited and
that's the reason why he invented the
reflecting telescope
as it encounters a reflecting telescope
a beam of light reflected from a
parabolic surface is reflected to a
single point at the focus of the
parabola without dispersion regardless
of color
and that's why ever since Isaac Newton
invented it the reflector telescope has
been the handiest tool of the optical
astronomer
of course light isn't the only wave in
the heavens nor the only kind of wave
that encounters a mirror and bounces
back
just as light waves reflect so do radio
waves
that's why the principle behind radar
and the telescope are one and the same
the dish of a radar reflector has the
same parabolic shape as the mirror in a
telescope
so whether it's the Coast Guard's
indispensable radar for 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 the object
from an eye chart at a few meters
to an incredibly distant star in the
reflecting lens of a modern telescope
light waves continue to review the most
amazing sites
and to a certain extent the journeys
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
brings light from the sun through empty
space to the Earth
what's waving
there being no obvious answer to that
question 19th century physicists at
least gave it a name
it was called The 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 transmits sound
throughout the 19th century a number of
ingenious experiments were done in an
attempt to detect the motion of the
Earth with respect to the luminiferous
ether
for one reason or another all of those
experiments failed
and 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 Through
The Ether
the story of that experiment
and its repercussions and consequences
is what we'll speak about when we meet
again next time
everything
thank you
[Music]
Annenberg media
for information about this and other
Annenberg media programs call 1-800
learner and visit us at www.learner.org
[Music]
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