The nature of light - FIU Faculty Websites

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Transcript The nature of light - FIU Faculty Websites

The nature of light
Newton’s Corpuscular Theory (1670)
According to Isaac Newton ( 1642 - 1727 ) in his
"Opticks" ( 1704 ) light is composed by
particles which are transmitted at a finite
speed by the "Luminiferous Aether Wind" , a
common or universal medium of propagation.
The Luminiferous Aether was imagined by
physicists since Isaac Newton as the invisible
"vapor" or "gas aether" filling the universe and
hence as the carrier of heat to our body and light
to our eyes.
Light and aether
How fast does the light
propagate?
The speed of light in a vacuum is defined to be exactly
299,792,458 m/sec, (approximately 186,282 miles per second).
Different physicists, including Galileo, have attempted to measure
the speed of light throughout history. We will discuss in detail the
experiment of Michelson and Morley (1887)
• The following phenomena
are commonly
observable:
• Reflection
• Rifraction
• Diffraction
Reflection:
Newton’s theory explained the phenomenon of reflection
by assuming that the light particles bounce on the mirror)
• Example: a ray of light hits the surface of
a mirror at an angle of 30^0 with the
surface. What is the angle that the light
forms with the mirror when it bounces
back?
• Example: a ray of light hits the surface of
a mirror at an angle of 30^0 with the
surface. What is the angle that the light
forms with the mirror when it bounces
back?
• Answ: 30^0
Reflection of light occurs when the light encounter a surface or
other boundary that does not absorb energy, and allows the
light to bounce away from the surface. The simplest example
of visible light reflection is the surface of a smooth pool of
water, where incident light is reflected in an orderly manner to
produce a clear image of the scenery surrounding the pool.
Throw a rock into the pool and the water is perturbed to form
waves, which disrupt the reflection by scattering the reflected
light rays in all directions.
The light goes from the sun, to the lake, to your eye, and
you perceive the images as coming from the lake.
Refraction.
• It is the "bending" of light at the interface of two materials
of different density. It accounts for the focusing action of
lenses. Atmospheric refraction (due to the curve of the
Earth's surface and the variation of atmospheric density
with height, in turn dependent on meteorological
conditions) is the effect which makes the Sun (or other
celestial body) look slightly higher in the sky than its true
astronomical position.
• The Refractive index (µ, MU): of a medium (or its index
of refraction) is defined as the ratio of the speed of light
in a vacuum to its speed in the medium.
Refraction
The corpuscular theory also explained the phenomenon of
refraction (the light particles change their direction when they
enter a medium of different density)
The denser is the medium, the more the light bends
The refraction of light is also responsible for the optical
phenomenon of mirage. There is no water in the sand;
what the image captured is the reflection of the sky!
The man sees the palms reflected in the sand …
• But this is an illusion. Cold air is denser than warm air and has
therefore a greater refractive index. As light passes from colder
air across a sharp boundary to significantly warmer air, the light
rays bend away from the direction of the temperature increase.
When light rays pass from hotter to colder, they bend toward
the direction of the temperature increase. If the air near the
ground is warmer than that higher up, the light ray bends in a
concave, upward trajectory.
• Once the ray reaches the viewer’s eye,
our brain interprets it as if it traces back
along a perfectly straight "line of sight". In
the previous image, an "inferior image" of
the sky above appears on the ground. The
viewer may incorrectly interpret this sight
as water which is reflecting the sky, which
is, to the brain, a more reasonable and
common occurrence.
Diffraction of light
• Newton found out that the white light from the Sun is
composed of light of different color, or spectrum (1670).
Each color cannot be further decomposed. If you put a
screen in front of the light decomposed by the prism
that lets only one color through ….
Each color cannot be further decomposed. If you put a
screen in front of the light decomposed by the prism that
lets only one color through ….
• This experiment was performed by Newton.
• Before him, it was believed that white light was colorless,
and that the prism itself produced the color. Newton's
experiments demonstrated that all the colors already
existed in the light and that "corpuscles" of light were
fanned out because particles with different colors
traveled with different speeds through the prism.
A rainbow is a phenomenon that causes a spectrum of
light to appear in the sky when the Sun shines on to
droplets of moisture (that act like prisms) in the Earth’s
atmosphere.
•
• These phenomena can also be explained
with Fermat's principle or the principle
of least time
• It is s the principle that the path taken
between two points by a ray of light is
the path that can be traversed in the
least time. This principle is sometimes
taken as the definition of a ray of light
• Pierre de Fermat ( 1601 or
1607 – 12 1665) was
a French lawyer at the
Parlement of Toulouse, France,
and … an amateur
mathematician.
• He made notable contributions
to analytic
geometry, probability,
and optics.
• He is best known for Fermat's
Last Theorem,
• The “theorem” says: there is no integer
numbers x, y and z for which satisfy the
“generalized Pythagoras theorem”
x^n+y^n=z^n when n=3 or larger.
• This property was proved only in 1995 by A.
Wiles, a professor at Princeton Univ; the
proof made international headlines and
Wiles won a million dollars prize for it.
The diffraction of light was hard to explain under
the assumption that the light is composed by
particles.
It can be explained by assuming that the
white light is composed by the
superposition of seven different waves
each with its own frequency, that we
perceive as colored lights
Back to the Fermat’s principle:
• Explaining the reflection with Fermat’s principle
is easy.
• Explaining the diffraction is easy too (light
particles of different colors travel with different
speed through the prism, and they separate
because they chose the path of least resistance)
• Explaining the diffraction is not so bad either:
the light travels and different speed in different
materials, and it bends because it chooses the
path of least resistance between the two
materials
• If a life guard on the beach has to run to rescue a person
drowning at sea, he won’t go on a straight line toward
the drowning person: he’ll run on the beach for the
longer stretch and swim for the shorter stretch because
he can run faster than he can swim.
• But Newton’t theory
could not explain why
a light ray that passes
though a pinhole
spreads out in circular
patterns …
Newton and Hooke
• R. Hooke suggested a wave theory of light in
his Micrographia (1665), comparing the
spreading of light vibrations to that of waves in
water. He suggested in 1672 that the vibrations
in light might be perpendicular to the direction of
propagation.
• Newton rejected a simple wave theory of light
because it could not account for rectilinear
propagation. It never occurred to him that the
waves of light might be very, very small.
Newton and Huygens
•
In 1678, Dutch physicist, Christiaan Huygens, believed
that light was made up of waves vibrating up and down
perpendicular to the direction of the light travels. This
became known as 'Huygens' Principle'. Huygen,
suggested that light wave peaks form surfaces like the
layers of an onion. In a vacuum, or other uniform
mediums, the light waves are spherical, and these wave
surfaces advance or spread out as they travel at the
speed of light.
• This theory explains why light shining through a pin hole
or slit will spread out rather than going in a straight line.
Newton's theory came first, but the theory of Huygens
better describes the motion of light. Huygens' principle
lets you predict where a given wavefront will be in the
future, if you have the knowledge of where the given
wavefront is in the present.
• Young’s Double-Slit Experiment definitely
confirmed that the light behaved as a
wave (1801). The light passes through
two closely spaced slits
…and that’s what you see…
This is an actual picture of light’s
interference.
Light has wavelike property
• Young’s Double-Slit Experiment definitely confirmed that
the light behaved as a wave (1801). The light passes
through two closely spaced slits….
• The alternating black and bright bands appearing on the
screen is analogous to the water waves that pass
through a barrier with two openings
Interference of waves
• This phenomenon is
called interference;
Viewing the light as a wave allows to
explain also the phenomena of reflection…
… and refraction. In the denser medium
the wave propagates at a lower speed.
…But it could not explain the
photoelectric effect
• In the photoelectric effect, electrons are
emitted from metals, liquid or gasses, as
a consequence of their absorption of
energy from electromagnetic radiations of
very short wavelength, such as … visible
light. Heinrich Hertz showed in 1887
that electrodes illuminated with ultraviolet
light create electric sparks more easily.
Photoelectric effect
• Light
Electrons
Dual properties of Light:
(1) waves and (2) particles
• Light is an electromagnetic radiation wave, e.g, Young’s
double slit experiment
• Light is also a particle-like packet of energy - photon
– Light particle is called photon
– The energy of phone is related to the wavelength of light
• Light has a dual personality; it behaves as a stream of
particle like photons, but each photon has wavelike
properties
• The photons have not been measured and isolate in
laboratory yet.