Download Slide 1 - Stanton

Light
1. Wave Vs. Particles
2. Electromagnetic Waves
3. Frequency and Wavelength
4. Light Vs. Sound
5. Space Travel & The Speed of
Light
6. Why Objects Have Color
7. Primary and Secondary Colors
8. Light Colors Vs. Pigments
9. The Electromagnetic Spectrum
10. Human production of visible
light
•Light Transmission
•Luminosity
•Polarized Light
•Coherent Light
•Lasers
•Holograms
Parts of the Ear
Light: Introduction
For centuries the nature of light was disputed. In the 17th
century, Isaac Newton proposed the “corpuscular theory”
stating that light is composed of particles. Other scientists,
like Robert Hooke and Christian Huygens, believed light to
be a wave. Today we know that light behaves as both a
wave and as a particle. Light undergoes interference and
diffraction, as all waves do, but whenever light is emitted, it
is always done so in discreet of packets called photons.
These photons carry momentum, but not mass.
Robert Hooke
Christian Huygens
Isaac Newton
Wave Vs. Particles
Light is an electromagnetic wave. As light travels through space
an electric field and a magnetic field oscillate perpendicular to
the wave direction and perpendicular to each other.
A light wave is transverse rather than longitudinal, since each
field oscillates in a plane perpendicular to the direction of the
wave. Unlike a pulse traveling down a length of rope, nothing is
physically moving in a light wave.
Light requires no medium! It can travel through space that
contains matter (such as air, glass, or water) or through a
vacuum.
- What would happen to Earth if light needed a medium?
Electromagnetic Waves
Electric and magnetic fields affect charges. Light is an
electric field coupled with a magnetic field. The two fields
oscillate together but in different planes. Visible light is just
a small part of the spectrum. The waves vary length and
frequency.
Wave Lengths of E.M
Spectrum
High vs. Low Frequency
Electromagnetic Waves (cont.)
In the top right picture, the blue wave represents an oscillating magnetic
field in the x-y plane. (Every point on this curve has an z coordinate of
zero.) It is a snapshot in time. Like the electric field, the magnetic field is
strongest at the crests and troughs.
Bottom right is shown an electric
and a magnetic field oscillating
together. This is an electromagnetic wave (light). The fields
travel through space together. They
have the same period and
wavelength, but they oscillate in
two different planes, which are
perpendicular to each other. The
electric field, the magnetic field,
and the wave direction are all
mutually perpendicular.
Frequency and Wavelength
The frequency of a light wave corresponds to the color we see. The
amplitude corresponds to brightness.
Light
Sound
Frequency
Color
Pitch
Amplitude
Brightness
Loudness
The frequency of visible light is extremely high compared to that of
audible sound. Red light, for example, is the lowest frequency of
visible light, but even red light has a frequency of over 400 trillion
Hertz. This means if you’re looking at a red light, over 400 trillion
full cycles of red light enter your eye every second! The frequency
of violet light is even higher—over 750 trillion Hz. Other types of
electromagnetic radiation, like X-rays, have even higher frequencies,
and some have lower frequencies, like radio waves. Just as our ears
are only capable of hearing certain range of sounds (20 – 20,000
Hz), our eyes can only see a small range of frequencies.
Frequency and Wavelength (cont.)
- Which end of the spectrum has more energy? How do you know?
High Frequency ↔ Small Wavelength
Low Frequency ↔ Long Wavelength
Vacuum speed is constant.
Light Vs. Sound
It is important to emphasize just how fast light is. Compared to light, sound is
a snail. A wise person once said, “Light travels faster than sound, which is
why some people appear bright until you hear them speak.” Have you ever
watched a baseball game from a distance? You see the batter make contact
with the ball, but the sound of the wallop is delayed. This is because,
although sound is really fast, light is super-duper fast. For all practical
purposes, when you see something is when it happened (at least for events
here on Earth). You can determine how far away a lightning strike is by
counting seconds from the time you see the lightning until you hear the
thunder. It takes sound about 5 s to travel a mile, so if the thunder lags
behind the lightning by 2 or 3 s, then the lightning strike occurred about half
a mile away.
Space Travel & The Speed of Light
We can’t always ignore the time light takes to travel. Whenever you
look into the night sky, for example, you’re really looking back into
time. The stars you see are so far away that the light they emit takes
years to reach us. Nearby stars are tens or hundreds light-years away.
A light-year is the distance light travels in one year, almost 6 trillion
miles. (Our sun is only about 8 light-minutes away).
https://youtu.be/EtsXgODHMWk
SOL= 299 792 458 m / s
Practice
• 1. In the 17th century, Isaac Newton proposed the “corpuscular
theory” Describe his theory.
• 2. Based on what you know, how would you explain the way
frequency and amplitude influence light?
• 3. If you where lost in space and had to contact a passing ship to be
rescued, what would be the best way to do this? Why?
Why Objects Have Color
Visible light is a combination of many wavelengths (colors), which
give it a white appearance. When light hits an object certain
wavelengths are reflected and others are absorbed. The reflected
wavelengths are the ones we see and determine the color of an
object.
In the first picture the tomato absorbs blue and green wavelengths
and reflects the red wavelength. In the second picture red light is
shone upon the tomato. The tomato is still reflecting the red
wavelength and thus still looks red. But in the 3rd picture blue light is
shone upon the tomato, and since the tomato absorbs the blue
wavelength the tomato appears to be black.
Primary and Secondary Colors
The primary light colors are Red, Blue, and Green (RGB).
The secondary light colors are Yellow, Cyan, and Magenta.
Combining pigments in painting is exactly the opposite:
The primary pigments are Yellow, Cyan, Magenta.
The secondary pigments are Red, Blue and Green.
Animation
Light Colors Vs. Pigments
Primary colors in light are red, green, and blue because when put together
in the right intensities they form white light. Televisions use this idea to
project pictures on the screen. When lights these colors are combined in
pairs they form the secondary colors for light.
Pigment colors are seen by reflected light. A primary pigment color is one
that absorbs only one primary light color and reflects the other two primary
colors. Thus yellow, magenta, and cyan are the primary colors for pigments.
Yellow reflects red & green, cyan reflects green & blue, and magenta
reflects red & blue. Secondary pigments colors then are blue, green, and
red because they absorb two primary light colors and reflect their own light
color back.
The Electromagnetic Spectrum
The electromagnetic spectrum covers a wide range of wavelengths and
photon energies. Visible light ranges from 400 to 700 nanometers.
About 550 nanometers, which is a yellowish green, is the wavelength to
which our eyes are most responsive. Only a small portion of the
electromagnetic spectrum is visible to us. The smaller the wavelength,
the more energy each photons of the light has.
Electromagnetic Spectrum (cont.)
Wavelengths other that visible light serve useful purposes:
Radio waves are very long (a few centimeters to 6 football fields) and can
be used to send signals. These signals are transmitted by radio stations.
They transmit information and music via amplitude modulation (AM) and
frequency modulation (FM).
Microwaves (a few millimeters long) are also used in communications.
Microwave ovens are great for heating food since food is primarily water, and
microwaves have just the right frequency to get water molecules vibrating.
Infrared (micrometers in length) are used in remote controls to change the
channel, and they are also radiated by objects that are warmer than their
surrounding (like your body). They make night vision equipment possible.
Ultraviolet light is harmful to our bodies because its wavelength is so
small. Short wavelength mean high energy for photons. UV causes our
skin to tan and burn. Fortunately, the ozone layer blocks most UV
radiation, but prolonged exposure to the sun should be avoided, since UV
rays can cause skin cancer. On the positive side UV radiation helps
people to produce their own vitamin D.
Electromagnetic Spectrum (cont.)
X-rays are even more energetic, and hence more dangerous, than UV
rays, but luckily they cannot penetrate our ozone layer. They are
produced in space and of course are used by doctors to get pictures of
your bones.
Gamma rays are the most energetic of the light waves and little is known
about them other than they are very harmful to living cells and are used
by doctors to kill certain cells and for other operations. They are
produced in nuclear explosions. Like other high energy rays, our
atmosphere protects us from gamma rays.
Astronomers have many different types of telescopes at their disposal to
observe the universe in all parts of electromagnetic spectrum. Some
telescopes are
ground-based; others
are space-based:
Luminous vs. Illuminated
A luminous object is a body that produces its own light such as
the sun or a light bulb.
An illuminated object is a body that reflects light, just like the
moon, people, and buildings.
Some objects, like water and glass, transmit light to some
extent. In order to be seen, light must come from an object one
way or the other.
Light Transmission
Transparent: Materials, such as window glass,
through which light can travel easily and through
which other objects can clearly be seen.
Translucent: Materials, such as glass blocks,
through which light can pass through but no clear
image can be seen.
Opaque: Materials which absorb and reflect light.
Objects cannot be seen through the material. Most
objects are opaque.
Thin Films & Thin Film Interference
The thin film effect refers to colors seen in
such things as soap bubbles and oil spills. It
occurs as a result of the constructive and
destructive interference of light waves, not
because of refraction as in a prism. When light
hits a bubble, some of it is reflected by the
outer (air-soap) interface (ray #1), while some
penetrates the bubble wall and is reflected by
the inner (soap-air) interface (ray #2). The two
reflected rays interfere with one another.
Typically, most wavelengths will be out of
Guinness Soap Bubble Records
phase since #2 has to travel a greater
distance than #1. However, one wavelength will be in
incident ray
phase and this corresponds to the color produced.
The color depends on how great the difference in
#1
distance is that the two rays travel, and this distance
#2
depends on bubble thickness. The variations in
reflected
thickness (thinner at the top, thicker at the bottom)
rays
are responsible for the different colors.
Continued on Next Slide
Soap Bubble Wall
Polarized Light
Light coming directly from the sun or
Electric Field
other sources is unpolarized, meaning
Orientations
the electric and magnetic fields oscillate
in many different planes. Polarized light refers to
light in which all waves have electric fields oscillating in the same plane.
Imagine trying to pass a large piece of sheet metal through the bars of a
jail cell. To do this you would have to orient the sheet vertically (or nearly
so), otherwise the bars would block the sheet. Here, the bars are
analogous to a polarizing filter, and the sheet is analogous to the plane
in which the electric field is oscillating.
A polarizing filter is made of a material with long molecules that allow
electromagnetic waves of one orientation through. If a wave has an electric
field with any other orientation, the filter will only allow a component to
pass through, absorbing the rest. Note that only transverse waves such as
light can be polarized. Much of the light we see is at least partially
polarized. For example, when light reflects off of surfaces it is partially
polarized. Some sunglasses contain polarizing filters which helps to block
glare (such as the glare that is noticeable when looking out over a lake on
a sunny day).
Coherent Light
Lamps, flashlights, etc… all produce light. But this light is released in
many directions, and the light is very weak and diffuse. In coherent
light the wavelength and frequency of the photons emitted are the
same. The amplitude may vary. Such things as lasers and
holograms are composed of coherent light.
Incoherent
Coherent
Lasers
Laser stands for light amplification by stimulated emission of
radiation. A laser is a device that creates and amplifies a narrow,
intense beam of coherent, monochromatic (one wavelength) light.
Here’s how they work.
There are 2 primary states for an atom, an excited state and a
ground state. The ground state is the lowest energy, most stable
state. In the excited state electrons are in a higher energy level. In a
laser, the atoms or molecules of a crystal (such as ruby) or of a gas,
liquid, or other substance are excited in the laser cavity so that more
of them are at higher energy levels (excited state) than are at lower
energy levels. When an excited electron drops back to a lower
energy level, a photon of a particular wavelength is released. This
photon stimulates other electrons to emit photons. All these photons
are in phase.
Holograms
As with any type of wave, light waves can interfere with one another.
The interference of two or more waves will carry the whole
information about all the waves. It is on this basis that holograms
work. Holograms make use of lasers and they work in the following
fashion: (Explanation on next slide.)
Beam Splitter
Laser
Reference Beam
Mirror
Object Beam
Beam Spreader
Light wave
interference
Film Plate
Object
Holograms
(cont.)
As the laser hits the beam splitter, it is split in two. The object beam heads
towards the object of interest, while the reference beam heads toward a
mirror. The beams are identical until the object beam shines on the object.
There some of the light is absorbed; some is reflected toward the film. After
reflecting off the mirror, the reference beam is reunited with the object beam
on the film. Because one beam interacted with the object and the other didn’t,
the two beams will be out of phase and interfere with one another. This
interference pattern is imprinted upon the holographic film plate, creating the
holographic image.
This pattern records the intensity distribution of the reflected light just as an
ordinary camera does. However, it also records the phase distribution. This
means that it contains information about where the waves are in their
oscillating cycles as they strike the film. To determine this the object beam
must be compared with the reference beam. This is accomplished via the
interference. Also unlike an ordinary photo, a hologram contains all its
information in every piece of it.
When viewed in coherent light the object appears in 3-D and viewing a
hologram from different angles will reveal the object from different angles.
Science History
• Before Galileo’s time (around 1600), many people believe that
light was infinitely fast. It’s so fast that it seemed like it took no
time to get from one place to another. Galileo and an assistant
went to the Italian countryside, a mile apart, and tried to measure
the speed of light by timing it. All they could determine was that
light is much faster than sound.
• Later that century (around 1667) a Danish astronomer named
Ole Roemer made the first accurate measurement of the speed of
light. He had been observing one of Jupiter’s moons, Io (which
Galileo had discovered). As Io circled Jupiter, it would be eclipsed
by Jupiter periodically. That is, Jupiter would block Io’s view from
Earth at regular intervals. Each time Io orbited Jupiter, an eclipse
would occur. The time between the eclipses was the period of
Io’s orbit. Roemer noticed that the eclipses sometimes took a
little longer, and sometimes they took a little less time. Io’s
period seemed to fluctuate: first Io would be behind schedule;
then it would be ahead of schedule. This pattern repeated itself
every year, which hinted to Roemer that the fluctuation had to do
with Earth’s motion around the sun.
Historical Background (cont.)
Because Jupiter is farther from the sun, it moves much slower
around the sun (recall Kepler’s third law). During the six-month
period depicted above, Earth is moving away from Jupiter.
Therefore, the light carrying the information of the eclipse took a
little longer to reach Earth, since Earth was “running away” from
that light. At the end of the six months, the light from Io had to
travel an extra distance about equal to the diameter of Earth’s orbit.
Roemer’s observed that Io eclipses were about 8 minutes behind
schedule after six months. Knowing approximately Earth’s orbital
diameter, Roemer calculated the speed of light at around 125,000
miles per second! Roemer’s speed, as great as it was, was actually
an underestimate. The true speed of light is just a half a smidgeon
under 3 · 108 m/s, which is about 186,300 miles per second! We
call this speed c. c = 2.9979  108 m/s  3  108 m/s
Historical Background (cont.)
• Roemer’s main contribution was proving that the speed of light is finite.
Since Roemer, several people contributed to determining the precise
value for c. In 1849 Louis Fizeau found an excellent approximation for c
without resorting to astronomical means. He used a rapidly rotating,
toothed wheel. He shined a beam of light through one opening between
the teeth, which reflected off a mirror over 5 miles away. When the
wheel spun fairly slowly, the light could easily pass through the opening,
reflect, and pass through it again in the other direction before its path
was blocked by the next tooth of the wheel. By making the wheel spin
faster and faster until the reflected beam of light was blocked, Fizeau
was able to calculate c.
• Jean-Bernard Foucault also made accurate measurements of c. He
shined light at a rotating mirror, which reflected to a stationary mirror,
back to the rotating mirror, and finally back toward the source. Because
the rotating mirror turned slightly while the light was traveling to the
stationary mirror and back, the rotating mirror reflected the light at a
slight angle. This angle allowed him to calculate c.
Michelson-Morely Experiment
Albert Michelson is best known for an experiment he did with Edward Morely
in 1887. At the time it wasn’t understood that light needed no medium
through which to travel. It was proposed that light traveled through an
invisible “ether” in space. The Michelson-Morely experiment was an attempt
to detect Earth’s motion through the ether. Here’s how it worked: First
imagine you’re standing still outside and there is a wind coming from the
north. If you run north, you’ll measure a greater wind speed. If you run south,
you’ll measure it slower. Whether you run north or south, though, you’ll still
feel the wind coming from the north. If you run east or west, however, not only
will the wind seem to change speed, so will its direction.
Now imagine a race between two equally fast swimmers. They each go the
same distance in a river, but one goes upstream and back while the other
goes directly across the river and back. With no current the race would
definitely be a tie, since their speeds and distances are the same. With a
current, however, the cross-stream swimmer will win. This is not obvious.
You should try to prove this. For a hint see the “river crossing--relative
velocities” slide from the presentation on vectors. It involves the same
principle as Michelson’s interferometer (but without lasers).
Michelson-Morely Experiment
Michelson-Morely Experiment (cont.)
Michelson built something called an interferometer
to try to measure a change in the speed of light in
two different directions. The Earth moving through
the ether around the sun is analogous to a wind or
current. Instead of racing two swimmers, Michelson
raced beams of light. Light was shone onto a mirror
that allowed half of it to pass through. Each beam
traveled the same distance before being reflected
back and allowed to recombine. Based on the
interference pattern of the combined waves,
Michelson should have been able to detect a
winner. But no matter how the experiment was
done, the race was always a tie. This eventually
forced physicist to abandon the ether theory.
Einstein resolved the problem in 1905 with his
theory of special relativity. In it he asserts that the
speed of light is the same no matter how fast or
which way an observer is moving.
Michelson
Einstein