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Most of the
things we see are visible because they re-emit light that strikes them from
another source
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Reflected
light is re-emitted back into the medium it came from
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Refracted
light is bent at the surface of a transparent medium and continues on through
in a straight line
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In 1650 Fermat proposed his principle of least time which
states that light takes the path that requires the shortest time
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For reflection
the shortest path and therefore the shortest time can be shown to be when the
path from the source of light and to the reflected image is equal
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This occurs
when the angle of the incident ray equals the angle of the reflected ray
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In a plane
mirror the image appears to
be coming from behind the mirror the same distance the object is in front of
the mirror
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This is called
a virtual image and can be explained by drawing rays of reflected light mirrored reflection
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Because the
front and back axis are reversed right and left appear reversed in the image
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For curved
mirrors the law of reflection applies and using ray optics we can see how
images are formed Concave mirrors
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Only part of
the light is reflected-up to 90% for a mirror
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Most
reflection occurs on rough surfaces and is called diffuse reflection-this
allows us to see most objects from any angle
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The surface is
said to be polished if there is little diffuse reflection
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A surface can
be polished for long wavelength radio waves yet very rough for light waves
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Refraction is
the bending of light as it hits a boundary obliquely between mediums where it has different speeds-as in
traveling from air into water
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The ray of
light takes a longer path but the time of travel is still the shortest because
more of the path is in the medium with the highest speed for light
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Light
refracts because as the wave front
encounters the surface of the water the side that hits the water first slows
down while the other side continues at a higher speed much like the wheels of a
car driving from a blacktop road into mud at an angle
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The car would
be pulled to one side as the wheel in the mud would slow down while the other wheel
continues on the blacktop for a short time at a higher speed
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The cause then
of refraction is the speed change of the wave fronts as they enter a new medium
at an angle
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They bend
toward the normal in going from a faster speed to a slower speed
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They bend away
from the normal in going from a slower speed to a faster speed
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Objects seen
obliquely under water appear to be nearer to the surface and displaced from
their actual location
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The thickening
earth’s atmosphere will bend light from the sun so we can still see it
after it has physical passed below our horizon
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Since rays
from the lower edge are bent more than from the upper edge the sun and the moon
appears somewhat elliptical just before setting or after rising
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A mirage is
not a reflection of the sky but refraction of light from the sky through the
warmer less-dense air near the road surface
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The wavy,
shimmering effect we see over hot objects is caused by the refraction of light
waves
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Light of
different frequency and color travel at slightly different speeds in a glass
prism with blue traveling about 1% less than red
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So blue light
is refracted the greatest and red the least
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A prism refracts light twice and the
effect is the spreading of white light into the color spectrum
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This is called
dispersion
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A rainbow is
formed when sunlight is refracted into a raindrop and after being reflected
from the back of the drop is again refracted out the front of the drop much
like a prism rainbow
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We see the
full spectrum when the angle between the rays to our eyes and the sun is
between 400 and 420
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The colors we
see come from many drops of water
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The rainbow is
actually a three dimensional cone that appears as a two dimensional semi-circle
in the sky
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The larger
secondary bow that we can sometimes see is caused by two reflections inside the
drop instead of one
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As we shine a
light from under water at an increasing angle to the normal of the surface we
will reach an angle where all the light will be reflected back into the water
and no light will emerge from the water
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This is called
the critical angle for total internal reflection to
begin to occur
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For a water to
air surface this angle is around 48o
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Total internal
reflection will occur in materials where the speed of light is less than the
speed of light outside
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A person under
water can see much of their surroundings by total internally reflected light
from the surface above
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Also the
entire 180 deg horizon can be seen through an angle of 96 degrees-twice the
critical angle because the light is refracted
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A lens that
compresses a wide view is called a fish-eye
lens
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Total internal
reflection occurs in glass with a critical angle around 43 deg
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Prisms provide
an excellent medium for changing the direction of light and for inverting
images in optical devices using internal reflections
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A pair of
prisms are used in binoculars to provide a longer path length for the light and
to invert the image to be right-side up
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This provides
for a much more compact device
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The critical
angle for a diamond is around 24.5 deg and is slightly different for each color
of light
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The sides of
the diamond are cut so most of the light that enters a diamond is internally
reflected and dispersed into colors and emitted at the top
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A diamond
appears to sparkle colored light
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Total internal
reflection also underlies the operation of optical fiber systems
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Optical fibers
are used in medical applications, many lighting applications and for
communication systems
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Optical cables
can carry more information clearer using light waves than the electric signals
used in copper wire
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Also the
optical cables can be much smaller
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The polar
bears fur is comprised of optical fibers
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Lenses use
refraction to bend light and to form images
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A lens that is
fatter in the middle converges the light to a point and is called a converging
or convex lens
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A lens that is
thinner in the middle diverges the light from a point in back of the
lens-diverging lens or a concave lens
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The first
cameras used a small pinhole to form an image on the film but with a small
amount of light
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Converging
lens help coin the phrase snap-shots
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We use a
converging lens as a magnifying glass when we hold the lens closer to the
object viewed than the focal length of the lens magnif
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This enlarged
image appears to be coming from in back of the lens and cannot be projected
onto a screen
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This is
therefore a virtual image
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When the
object viewed through a convex lens is further than the focal length from the
lens a real image is formed that can be projected unto a screen real image
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The real image
will always be inverted from a convex lens and the size will depend on the
distance of the object from the focal point
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A diverging
lens will always produce a smaller,upright, and virtual image of the object concave lens
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It is used as
a finder lens on a camera to simulate the actual photograph
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Spherical
aberration is distortion caused by the curvature of the lens
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Chromatic
aberration is distortion caused by the different colors of light refracting differently
in the lens
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Using
combinations of lenses can correct these defects
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The pupil of
the eye controls the amount of
light that enters the eye eye
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When the pupil
is small we can see better because the light enters only the center of the
eye’s lens where the above aberrations are a minimum
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We see better
in bright light because our pupils are smaller
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Astigmatism of
the eye is a defect where the cornea is curved more in one direction than the
other and sharp images are not formed in the eye astig
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This can be
corrected with lenses that have more curvature in one direction than another
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Another option
today is corrective surgery on the shape of the cornea