Transcript ISNS3371_040307_bw - The University of Texas at Dallas

ISNS 3371 - Phenomena of Nature
Secondary Rainbows
ISNS 3371 - Phenomena of Nature
The angle at which the
red light leaves the
raindrop measured from
the anti-solar point for
the double
reflection/secondary
rainbow is 51° as
compared with 42° for
the single
reflection/primary
rainbow. So the
secondary rainbow
encircles the primary
rainbow at about 10°
farther from the anti-solar
point (with reversed
colors).
ISNS 3371 - Phenomena of Nature
The light of the secondary bow is one-tenth the intensity of that of the
primary bow, given the same viewing conditions.
ISNS 3371 - Phenomena of Nature
Polarization
Light emitted by the sun, a lamp in the classroom, a candle
flame, etc… is unpolarized light - created by electric charges
which vibrate in a variety of directions - an electromagnetic
wave (transverse) which vibrates in a variety of directions.
Helpful to picture unpolarized light as a wave which has an
average of half its vibrations in a horizontal plane and half of its
vibrations in a vertical plane.
Polarized light waves - light waves in which the vibrations occur in a single
plane.
Polarization - Process of transforming unpolarized light into polarized light.
Most common method of polarization uses a
Polaroid filter - made of a special material
capable of blocking one of the two planes of
vibration of an electromagnetic wave. When
unpolarized light is transmitted through a
Polaroid filter, it emerges with one-half the
intensity and with vibrations in a single plane;
it emerges as polarized light.
ISNS 3371 - Phenomena of Nature
Two filters with polarization axes perpendicular to each other will
completely block the light.
Light is polarized upon passage through the first filter - say, only vertical
vibrations were able to pass through. These vertical vibrations are then
blocked by the second filter since if its polarization filter is aligned in a
horizontal direction.
Like picket-fence and standing wave on a rope - vibrates in a single plane.
Spaces between the pickets of the fence allow vibrations parallel to the
spacings to pass through while blocking vibrations perpendicular to the
spacings.
Orient two picket fences
such that the pickets are
both aligned vertically vertical vibrations will pass
through both fences align pickets of second
fence horizontally - the
vertical vibrations which
pass through the first
fence will be blocked by
the second fence.
ISNS 3371 - Phenomena of Nature
Polarization by Reflection
Unpolarized light can also undergo polarization by reflection off of nonmetallic
surfaces - extent dependent upon the angle at which the light approaches the
surface and upon the surface material.
Metallic surfaces reflect light with variety of vibrational directions - unpolarized.
Nonmetallic surfaces (asphalt, snow, water,
paint on a car) reflect light such that there is a
large concentration of vibrations in a plane
parallel to the reflecting surface. A person
viewing objects by means of light reflected off
of nonmetallic surfaces will often perceive a
glare if the extent of polarization is large.
Which pair of glasses is best suited for
automobile drivers, fishermen, snow skiers?
ISNS 3371 - Phenomena of Nature
Adding a third filter with between
two filters polarization axis at 45º to
the other two will allow light though.
How?
Remember, unpolarized light vibrates in all different directions. So not just
the light with horizontal vibrations passes through the first filter, but all light
with a vibrational component in the horizontal direction - in other words, all
but the light with vertical vibrations has some component in the horizontal
direction that gets through.
ISNS 3371 - Phenomena of Nature
Before the middle filter, the light is
horizontally polarized.
The component of horizontally polarized
light along 45º gets through the middle
filter.
The component of that light in the vertical
direction then gets though the last filter.
ISNS 3371 - Phenomena of Nature
When light passes through a transparent material such as plastic, internal
(and normally invisible) stresses in the material can rotate the angle of
polarization - different colors will be rotated by different amounts. Place
horizontal polarizer below the object, and a vertical one above it - no light will
be transmitted unless there are stresses inside the object that rotate the light.
Useful in the design of engineering structures - build a model out of plastic,
and view it with crossed polarizers. Put a force on the model - regions of the
model that are stressed the most will show up in color. This way you can
determine which parts of the structure are most likely to break, and the design
can be changed (if necessary) to relieve some of that stress.
ISNS 3371 - Phenomena of Nature
Lenses and Mirrors
(and applications to astronomy)
ISNS 3371 - Phenomena of Nature
Geometry of a Concave Mirror
Focus
Principal axis
Vertex
Focal length
Center of curvature - the center of the circle of which the mirror represents a
small arc
Principal axis - a radius drawn to the mirror surface from the center of
curvature of the mirror - normal to mirror surface
Focus - the point where light rays parallel to principal axis converge; the focus
is always found on the inner part of the "circle" of which the mirror is a small
arc; the focus of a mirror is one-half the radius
Vertex - the point where the mirror crosses the principal axis
Focal length - the distance from the focus to the vertex of the mirror
ISNS 3371 - Phenomena of Nature
Geometry of a Converging (Convex) Lens
Focus
Optical axis
Focal length
Optical axis - axis normal to both sides of lens - light is not refracted
along the optical axis
Focus - the point where light rays parallel to optical axis converge; the
focus is always found on the opposite side of the lens from the object
Focal length - the distance from the focus to the centerline of the lens
ISNS 3371 - Phenomena of Nature
Image Magnification Using a Simple Lens
L2
D1
D2
L
D
_1
_1
=
L2
D2
Focal Plane
L1
ISNS 3371 - Phenomena of Nature
The image formed by a single lens is inverted.
ISNS 3371 - Phenomena of Nature
The Eye
The eye consists of pupil that allows light into the eye - it controls the amount
of light allowed in through the lens - acts like a simple glass lens which
focuses the light on the retina - which consists of light sensitive cells that
send signals to the brain via the optic nerve. An eye with perfect vision has its
focus on the retina when the muscles controlling the shape of the lens are
completely relaxed - when viewing an object far away - essentially at infinity.
ISNS 3371 - Phenomena of Nature
When viewing an object not at infinity, the eye muscles contract and
change the shape of the lens so that the focal plane is at the retina (in an
eye with perfect vision). The image is inverted as with a single lens - the
brain interprets the image and rights it.
ISNS 3371 - Phenomena of Nature
Types of Optical Telescopes
ISNS 3371 - Phenomena of Nature
Magnification Using Two Lenses - Refracting Telescope
f1 = 0.5 m
f2 = 0.1 m
f1 = 0.5 m
f2 = 0.3 m
Refracting telescope - consists of two lenses - the objective and the
eyepiece (ocular). Incident light rays (from the left) are refracted by the
objective and the eyepiece and reach the eye of the person looking through
the telescope (to the right of the eyepiece). If the focal length of the
objective (f1) is bigger than the focal length of the eyepiece (f1), the
refracting astronomical telescope produces an enlarged, inverted image:
magnification = f1 /f2
ISNS 3371 - Phenomena of Nature
Refracting Telescope
Uses lens to focus light from
distant object - the eyepiece
contains a small lens that
brings the collected light to a
focus and magnifies it for an
observer looking through it.
ISNS 3371 - Phenomena of Nature
The largest refracting telescope in the world is the at the University of
Chicago’s Yerkes Observatory - it is 40 inches in diameter and 63 feet long.
ISNS 3371 - Phenomena of Nature
Reflecting Telescope
The primary mirror focuses light at the prime focus. A camera or another
mirror that reflects the light into an eyepiece is placed at the prime focus.
ISNS 3371 - Phenomena of Nature
Types of Reflecting Telescopes
Each design incorporates a small mirror just in front of the prime focus to
reflect the light to a convenient location for viewing.
ISNS 3371 - Phenomena of Nature
The Keck Telescopes
Largest in the world - on Mauna Kea in Hawaii. 36 hexagonal mirrors
function as single 10-meter mirror.
ISNS 3371 - Phenomena of Nature
The Hubble Space Telescope
The Hubble Space Telescope is 43.5 ft long and weighs 24,500 lbs. Its
primary mirror is 2.4 m (7 ft 10.5 in) in diameter.
ISNS 3371 - Phenomena of Nature
Refracting vs Reflecting Telescopes
Reflecting telescopes are primary astronomical tools used for research:
1. Lens of refracting telescope very heavy - must be placed at end of
telescope - difficult to stabilize and prevent from deforming
2. Light losses from passing through thick glass of refracting lens must be very high quality and perfectly shaped on both sides
3. Refracting lenses subject to chromatic aberration
ISNS 3371 - Phenomena of Nature
Lens and Mirror Aberrations
SPHERICAL (lens and mirror)
Light passing through different parts of a lens or reflected from
different parts of a mirror comes to focus at different distances from
the lens.
Result: fuzzy image
CHROMATIC (lens only)
Objective lens acts like a prism.
Light of different wavelengths (colors) comes to focus at different
distances from the lens.
Result: fuzzy image
ISNS 3371 - Phenomena of Nature
Chromatic Aberration in Lenses
Focal point
for blue light
Simple lenses suffer from
the fact that different colors
of light have slightly
different focal lengths. This
defect is corrected by
adding a second lens
The problem
Focal point
for red light
Focal point
for all light
The solution
ISNS 3371 - Phenomena of Nature
Spherical Aberration in Lenses
Simple lenses suffer
from the fact that light
rays entering different
parts of the lens have
slightly difference focal
lengths. As with
chromatic aberration,
this defect is corrected
with the addition of a
second lens.
The problem
One focal point
for all light rays
The solution
ISNS 3371 - Phenomena of Nature
Spherical Aberration in Mirrors
The Problem
Simple concave mirrors suffer
from the fact that light rays
reflected from different locations
on the mirror have slightly
different locations on the mirror
have slightly different focal
lengths. This defect is corrected
by making sure the concave
surface of the mirror is parabolic
The Solution
All light rays converge
at a single point
ISNS 3371 - Phenomena of Nature
The image from an reflecting telescope is inverted.
Focus Inversion Animation
The focus is adjusted by changing the secondary mirror position.
Mirror Position and Focus Animation
ISNS 3371 - Phenomena of Nature
Uses of Telescopes
1.
Imaging
–
use a camera to take pictures (images)
–
Photometry  measure total amount of light from an object
2.
Spectroscopy
–
use a spectrograph to separate the light into its different
–
wavelengths (colors)
3.
Timing
–
measure how the amount of light changes with time
(sometimes in a fraction of a second)
ISNS 3371 - Phenomena of Nature
Imaging
• In astronomy, filters are
usually placed in front of
a camera to allow only
certain colors to be
imaged
• Single color images are
superimposed to form
true color images.
ISNS 3371 - Phenomena of Nature
Nonvisible Light
•
•
•
Most light is invisible to the human eye - gamma rays, x-rays, ultraviolet,
infrared, radio waves.
Special detectors/receivers can record such light - each type of light can
provide information not available from other types.
Digital images are reconstructed using false-color coding so that we can see
this light.
Chandra X-ray image of the Center of the Milky Way Galaxy
ISNS 3371 - Phenomena of Nature
The Crab Nebula
Visible
Radio Waves
Infrared
X-rays
ISNS 3371 - Phenomena of Nature
Atmospheric Effects
Earth’s atmosphere causes problems for astronomers on the ground:
• Bad weather makes it impossible to observe the night sky.
• Man-made light is reflected by the atmosphere, thus making the night
sky brighter.
– this is called light pollution
• The atmosphere absorbs light - dependent on wavelength
• Air turbulence in the atmosphere distorts light.
– That is why the stars appear to “twinkle”.
– Angular resolution is degraded.
ISNS 3371 - Phenomena of Nature
Atmospheric Absorption of Light
• Earth’s atmosphere absorbs most types of light.
– good thing it does, or we would be dead!
• Only visible, radio, and certain IR and UV light make it through to the
ground.
To observe the other wavelengths, we must put our telescopes in space!
ISNS 3371 - Phenomena of Nature
Space Astronomy
ISNS 3371 - Phenomena of Nature
Space Based Telescopes
Chandra X-ray Obs.
FUSE (Far UV)
Compton Gamma Ray Obs.
Hubble Space Telescope
Spitzer Space Telescope (IR)
ISNS 3371 - Phenomena of Nature
Radio Telescopes
• The wavelengths of radio waves are long.
• So the dishes which reflect them must be very large to achieve any
reasonable angular resolution.
305-meter radio telescope at Arecibo, Puerto Rico