Download Part5-Electromagneti..

A Brief History of Light
Isaac Newton, 1600’s: “Light is like little bullets.”
•
Scientists: “Okay, right, that makes sense!”
Thomas Young, 1800’s: “No, no, check this out.” (Shines
light through two slits.) “Interference! It’s a wave!”
Scientists: “Oooooh, you’re right, it can’t be little
bullets if it can do that!”
Albert Einstein, 1905: “No, no, check this
out.” (Describes the photoelectric effect with
“photons”.) “It’s quantized! It must be particles!”
Scientists: “Oooooh, you’re right! Waves don’t
work that way!”
(Einstein gets a Nobel prize in 1921.)
Prince Louis deBroglie, 1924: “Hey, everybody.
Maybe particles are waves! Everything’s both!”
Scientists: “..... Woah.”
(deBroglie gets a Nobel Prize in 1929.)
5: Electromagnetic Waves
(Chapters 33 & 34)
•
Phys130, A01
Dr. Robert MacDonald
•
•
2
Snapshot of a light wave
Wave vs Particle
Shape of the wave at a certain moment in time.
electric
field
...
photon
Pirate and Ninja Bunnies from “Bunny” (http://www.frozenreality.co.uk/comic/bunny/)
P
Models of Light
Light is obviously very important to us generally as
human beings. It’s also very important to us as
engineers and scientists.
History graph
of a light wave
So there are many different models of light — some
more simplified or abstract than others — that we can
use depending on the situation.
History of the fields at
point P as the wave passes.
• The main types of models are particles (photons),
waves, or rays.
E = electric field
B = magnetic field
We’ll be starting with the “ray” model. This branch is
often called geometric optics.
We’ll come back to the wave model later. That
branch is often called physical optics.
Rays
A point source of light:
A light ray represents an imaginary line along the
direction of travel of the light. (Or maybe the path of
a single photon, if you want to think of it that way.)
A light ray will continue to travel along a straight line
until it interacts with something else. Possible
interactions include:
• Reflection
• Refraction (bending)
• Scattering
• Absorption
happen at an interface
between two materials
happen within a
single material
7
Rays go out uniformly
in all directions.
Extended source of light, such as a flood light.
(A source that has some shape to it).
An object that isn’t glowing.
Light radiates uniformly in all
directions from each point on
the surface of the light source.
Reflected light radiates
in all directions from
each point on the surface
of the light source.
No real difference between
this and the floodlight!
Light from a point source going past an object.
Light from a point source going past an aperture (hole).
screen
point
source
Some rays are
blocked.
Shadow matches the
object.
Shadow’s edges are
sharp.
screen
point
source
Some rays are
blocked.
Light patch matches
the hole.
Light patch edges are
sharp.
Light from an extended source going past an
aperture.
screen
Light from an extended object going past a pinhole.
screen
Some rays are
blocked.
extended
source
Light patch matches
the hole.
Light patch edges are
blurred.
Images
image!
Most rays are blocked.
Only “one” ray from
each place gets
through the pinhole.
Image forms on
screen!
Note that if the hole isn’t of zero size, the image will blur.
Seeing an Object
To get an image on a screen, the light from a given point on the
object (or other source of light) must strike exactly one point on
the screen.
screen
eye
image!
not an
image
15
Light rays from each point on the object go
everywhere. Some light from each point reaches the
eye.
16
fog
eye
eye
If the light from the same point reaches the eye from
several different directions — e.g. it’s scattered by fog
— then we see each point fuzzed out, and object
appears blurry.
As long as only one ray from each point on the object
reaches the eye, we see the object clearly.
17
18
screen
screen
projector
projector
eye
Light from an image on a screen is scattered in all
directions. We see the image in the same way as we see
any object. It’s a sort of copy of the object. The eye
sees no difference between an object and an image.
19
eye
Light from an image on a screen is scattered in all
directions. We see the image in the same way as we see
any object. It’s a sort of copy of the object. The eye
sees no difference between an object and an image.
20
object
virtual image
So now you know:
• What the “ray” model of light is.
• How the ray model can describe what will appear on
a screen.
eye
When we look in a mirror, the rays that reach the eye
appear to come from an object behind the mirror.
What we see is called a virtual image.
• What an image is, and how an image is formed.
• What a “real” image and a “virtual” image are.
As far as the eye is concerned, it’s the same as a real
image (or an object). But the light that we see is not
coming from the virtual image, unlike a real image.
21
Polarization
A string can have transverse waves in the up-anddown (vertical) direction, or in the side-to-side
(horizontal) direction.
Waves that are only “waving” in one direction are said
to be polarized.
Up-and-down waves are vertically polarized.
Side-to-side waves are horizontally polarized.
If you sent a “twist” along the string, that would be an
unpolarized wave.
If you ran the twisty wave through a vertical slot,
the string on the other side could only wave
vertically.
•
•
•
22
Light is a transverse wave, as well.
• Can also be horizontally or vertically polarized (or
neither).
• Light’s “polarization” is the direction its electric field
is oscillating.
Most visible light is unpolarized when created.
• It can be polarized by a “polarizing filter” —!like the
slot for the string wave.
“Unpolarized” light is made up of a mixture of all possible
polarizations.
Be warned: lots of vocabulary ahead!
24
Polarizing Filters
Picturing polarization
A good, and very common, polarizing
filter is Polaroid, used in sunglasses and
camera filters.
Fig. 33.9
Horizontally-polarized light is absorbed
by the Polaroid.
Fig. 33.10
•
The electric field in the light pushes
electrons back and forth on long
molecules in the Polaroid (like a little
tiny BBQ grill), absorbing the light’s
energy and producing heat.
•
The vertically polarized light has
nowhere to push the electrons, so it
just goes right through.
An “ideal” filter passes 100% of the light
with the right polarization, and none of
the perpendicularly-polarized light.
Intensity After Polarization
The electric field vector of a photon can be
decomposed into vertical and horizontal components.
Unpolarized light is made up of all possible
polarizations. So on average the horizontal and vertical
components are equal.
Polarizers select one component and absorb the other.
So half the light gets through, and the resulting light
beam has half the intensity of the incident light
beam.
• This is only true if the incident light is unpolarized.
27
Fig. 33.11
Polarized Incidence
What happens when polarized light hits a polarizer?
• A polarizer is often called an analyzer when it’s used
with already-polarized light.
If the incident light is polarized in the same direction as
the analyzer’s polarization axis, the light goes right
through (for an ideal filter!).
If the incident light’s polarization is perpendicular to the
analyzer’s polarization axis, the light is entirely blocked.
So the important relationship is the angle between the
light and the polarizer’s axis.
28
For incident light at some
angle θ, we can decompose
the incident polarization.
If the amplitude of the electric field of the incident light
is E, then the amplitude of the electric field that makes it
through the analyzer is E cosθ.
• Only the component
• θ=0 means the light is polarized in the same
aligned with the analyzer
is passed.
direction as the analyzer. ! It all gets through.
cos(0) = 1
• θ=π/2 (90º) means the incident light is
perpendicular to the analyzer,. ! It’s all absorbed.
cos(π/2) = 0
Fig. 33.12
The “analyzer” is called an analyzer because you can turn
it until you get zero light coming through; then the
polarization of the incident light is perpendicular to the
analyzer’s polarization axis.
29
30
Intensity after two polarizers
Example: Two Polarizers
Just like with other waves, the intensity of an
electromagnetic wave (a light wave) is proportional to
the square of the electric field amplitude: I α E2.
Incident light with intensity I0 shines through two
consecutive ideal polarizers. The angle between the
polarization axes of the two polarizers is π/4 (45º).
We can use this to compare the intensity before and
after polarization. Take a ratio to get rid of constants:
What is the intensity of the light after each polarizer?
“Malus’s Law” for polarized light
passing through another polarizer.
(a.k.a. “the cosine-squared law”)
(Remember that for unpolarized light, I = I0/2.)
32
Example: Three polarizers
Angle between the first
and second sheets:
θ12 = 60° –!0° = 60°
Angle between the
second and third sheets:
θ23 = 90° –!60° = 30°
Liquid Crystal Display (LCD)
Reflection and Refraction
Reflection is when light bounces off of a surface. A
“surface” here means the interface between any two
media (air and glass, for example).
Reflector or back light
Horizontally polarizing filter
Refraction is when light enters a new medium. Its
path is usually bent in the process.
Electrodes
Liquid crystals
Usually you get a little bit of both at the same time.
• Consider light travelling through air (a medium) and
striking a glass window (another medium)...
Electrodes
Vertically polarizing
filter
Diagram
from Wikipedia.
36