Download Polarization

Friday lunchtime was the equinox. Which of the
following will NOT happen between now and Mid
June?
A. 
B. 
C. 
D. 
Earth will get closer to the Sun
The North Pole will tilt toward the sun
Winter will arrive in Chile
The south pole will get less daylight
Lecture 15
Applications in Optics
(Giancoli Chapter 23/25)
What is this picture?
A.  Map of the early universe in Polarized light
B.  Graph of the stress in a piece of glass using polarized light
C.  Plot of the optical aberrations in a human eye
Human Vision
Contact lenses & eyeglasses
Cameras
Telescopes
Microscopes
Refraction
Spectrographs
Dispersion
Liquid Crystal Displays
Diffraction
Antireflection coatings
Polarization
X-ray “optics”
Lenses
Mirrors
Focal Length
Magnification
Polarization
•  Light is polarized when its electric
fields oscillate in a single plane.
•  Consequence of the direction of
oscillation of electrons in atoms
•  Light gets polarized when it reflects
off surfaces, or is scattered by gases
(e.g. the atmosphere)
•  Any electromagnetic wave can be
polarized.
•  Another important example is radio
waves.
• Can sound waves be polarized?
Polarizing Filters (polaroid)
•  Polarized light will not be transmitted through a polarized film whose axis
is perpendicular to the polarization direction.
•  Polarizing filters are used in Photography, Sunglasses, Science Labs etc.
•  Usually created by stressing plastics while they are still partially melted,
creating a preferred direction of vibration for the molecules.
Transmitted
Blocked
Many Car windows show a pattern when viewed
though polarized sunglasses.
The glass contains stresses created by heating and cooling.
It is designed to shatter into tiny pieces in an accident, instead of
sharp shards that could cut people .
The stresses give the glass molecules preferred directions of
oscillation, hence they act as a polarizing filter.
Crossed Polarizers
If initially unpolarized light passes through
crossed polarizers, no light will get through
the second one.
This can be very useful, as certain special
materials can rotate the plane of polarization.
Polarization
When light passes through a polarizer, only the
component parallel to the polarization axis is
transmitted. If the incoming light is planepolarized, the outgoing intensity is:
You are looking through a single
polarizer at an unpolarized light.
What angle do you need to rotate a
second polarizer (relative to the
first) in order to block half of the
remaining light?
A. 
B. 
C. 
D. 
0 degrees
30 degrees
45 degrees
90 degrees
Polarization upon Reflection
•  Light is also polarized after reflecting from a nonmetallic surface.
•  At a special angle, called the polarizing angle or Brewster’s angle, the
polarization is 100%.
•  Reflected Light is Horizontally Polarized
• Incident waves that are vertically polarized get absorbed and so are not
reflected
(24-6a)
For light in air,
reflecting off water,
Brewster’s angle is
53 degrees
Use of Polarizer to remove Reflections and
Glare in Photography (and Everyday life)
Without polarizer:
(or adjusted horizontally)
Reflections in surface of water
With Polarizer:
(adjusted vertically)
Reflections suppressed.
The leaves at the bottom of the
pond are now visible
Polarization of rainbows
•  Rainbows involve reflection and
refraction both inside and at the
boundaries of the raindrops.
•  The rainbow is 100% linearly
polarized
•  The polarization axis is radial to the
bow
•  Just rotate your head while
wearing polarized sunglasses
Why is the Sky Blue? and is it also Polarized?
•  The clear sky is blue due to Scattering by
air molecules.
•  Scattering is a directional process.
•  Examine the sky with polarized
sunglasses!
•  At 90 from the Sun, the polarization is
upto 80%. -so it will look dark!
The blue sky is polarized
Rotating your polarized sunglasses will show regions of light and dark in
the sky, that change depending on where you look. Light from other
objects in the sky are mostly not polarized and so the moon, planets and
brighter stars can sometimes be seen in datytime using this trick.
The polarization pattern is quite striking
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Polarization of the sky reaches
its maximum strength at 90
degrees from the sun
Go see if you can observe the
overall pattern for yourself
Its very hard to photograph!
The Sun is hidden behind a
streetlamp in this photo (I had to
crouch dwn and position the
camera very carefully!)
Map of the polarization pattern in the Cosmic
Microwave Background
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• 
• 
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Found by the South pole telescope and announced this week!
It confirms a strange and ridiculously specific prediction of the Big Bang theory
The most significant cosmological discovery in a generation ?
It looks a lot like a car window viewed though a polarizer
The “stress” pattern in this case was imprinted by gravitational waves during the
earliest moments of the big bang
South Pole Telescope
Liquid Crystal Displays (LCD)
Color LCD displays are more complicated; each pixel
has three subpixels to provide the different colors. A
source of light is behind the display (unlike calculators
and watches, which use ambient light). The pixels must
be able to make finer adjustments than just on and off to
provide a clear image.
25.1 Cameras, Film, and Digital
A digital camera uses CCD sensors instead of
film. The digitized image is sent to a processor for
storage and later retrieval.
Cameras, Film, and Digital
Basic parts of a camera:
•  Lens
•  Light-tight box
•  Shutter
•  Film or electronic
sensor
Cameras, Film, and Digital
There is a certain range of distances over which
objects will be in focus; this is called the depth
of field of the lens. Objects closer or farther will
be blurred.
The Human Eye
The human eye resembles a (vastly more
complex and sophisticated) camera in its
basic functioning, with its adjustable lens, iris
(aperture), and retina (detector).
The Human Eye; Corrective Lenses
Nearsightedness can be corrected with a diverging lens.
And farsightedness with a converging lens.
Underwater vision and goggles
Vision is blurry underwater because light rays
are bent much less than they would be if
entering the eye from air. This can be avoided by
wearing goggles.
Why is it that when you open your eyes under
water (with no goggles!), everything is
Blurry?
A.  Your eyes cannot focus under water
B.  The refractive index of water is similar to that of your eye’s
lens.
C.  The water is rippling, blurring the view
D.  It stings, so you never noticed!
A nearsighted person requires what type of
lenses
In their eyeglasses or contact lenses ?
A. 
B. 
C. 
D. 
Converging
Diverging
Depends on their prescription
Neither
Limit of Resolution: Diffraction
•  Resolution is the smallest separation (or
angle) at which a lens can barely distinguish
two separate objects.
•  Resolution is limited by aberrations and by
diffraction.
•  Aberrations can be minimized.
•  Diffraction is unavoidable; it is due to the
size of the lens compared to the wavelength
of the light.
Limit of Resolution for Circular Apertures
For a circular aperture of diameter D, the central
maximum has an angular width:
Rayleigh Criterion, or the Diffraction Limit
The Rayleigh criterion states that two images are just resolvable when the
center of one peak is over the first minimum of the other.
•  The chief practical implication of Rayleigh’s Criterion, is that ANY
optical device (eye, camera, telescope, radio-telescope etc) cannot
resolve details finer than θ (in radians) = 1.22λ/D. Where D is the diameter
(in meters) of the objective lens or mirror, and λ the wavelength (in
meters) of the light in question.
•  Similarly, light emitted from an aperture D will spread out with this angle
(θ), due to diffraction. Even if the light rays start out perfectly parallel!
•  Thus, the Wider the Aperture, the Sharper the Image. Remember this!
What is the smallest detail on the Earth’s surface that can be made out in
photos taken by a camera with a 2.3m diameter objective mirror, carried by
on a satellite in near earth orbit?
The Hubble Space Telescope (diameter 2.3 m is similar to the imaging satellites flown
by the National Reconnaissance Office (NRO), except they look DOWN!
What’s the smallest detail the human
eye can see at a distance of:
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20 cm?
1 meter ?
10 meters
1 km?
100 km ?
500,000 km ?
Why can’t we see bacteria without a microscope?
Why can’t you recognize people a mile away?
Spherical Aberrations of Lenses
Spherical aberration: rays far from the lens axis
do not focus at the focal point.
Solutions: compound-lens systems (camera lenses can have > 15 elements!)
use only central part of lens (e.g. by stopping it down)
Aspherical lens surfaces (expensive to produce)
Aberrations of Lenses and Mirrors
Geometric Distortion: caused by variation in
magnification with distance from the lens. Barrel
and pincushion distortion:
Solutions: multiple elements, aspheric curves, stopping down, image
processing
Chromatic Aberration
Light of different wavelengths has different indices of refraction and focuses
at different points
Solutions: Use only the center, stop down, use very long focal length,
use colored filters, use multiple lenses
The Achromatic Doublet
•  Achromatic doublet is a lens made of two lenses of different glass types
that have different amounts of dispersion.
•  Usually a Strong Converging lens made from a low dispersion glass, is
glued to a Weaker Diverging lens (made from a higher dispersion glass)
•  The space between can be filled with glue, or oil.
•  Anti-reflection coatings are required to prevent “ghost” images forming
Refracting Telescopes
•  Refractors consist of an objective lens and an eyepiece lens.
•  Naturally produce upside down images
•  A terrestrial telescope, used for viewing objects on Earth, should produce
an upright image.
•  Here are two models, a Galilean type and a spyglass:
What is the magnification of a
telescope (f=1000mm) when used
with a 25mm eyepiece ?
A. -40
B. 250
C. -500
D. 1000
Reflecting Telescopes
•  Astronomical telescopes need to gather as much light as possible.
•  And obtain the sharpest view possible.
•  The objective must be as large as possible.
•  Mirrors are now used instead of lenses, as they can be made much larger.
•  Mirrors focus all wavelengths of light equally (no chromatic aberration)
Newtonian
Cassegrain
Gemini telescope objective mirror is 8
meters in diameter.
The next generation will be 30 m !