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12/13/2012
1
Notes: Light
and Optics
Light goes straight
• Light travels in a straight line unless
it interacts with a medium.
• The material through which a wave
travels is called a medium.
• Light can be reflected, refracted
(bent), scattered, or absorbed.
Reflection
• Reflection: when an object or wave
bounces back off a surface through
which it cannot pass.
Law of Reflection
• The law of reflection states that the
angle of incidence equals the angle
of reflection.
Video
Two Types of Reflection
Refraction
• Refraction: the bending of light
waves due to change in speed.
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Refraction
• Refraction: when light rays enter a
medium at an angle, the change in
speed causes the rays to bend, or
change direction.
• Refraction is what causes white light
to break into colors like in a rainbow.
Prisms
Rainbows Light & Matter
• Transparent objects let light through.
• Translucent objects scatter light.
• Opaque objects reflect or absorb light.
Color of Light
• Color of Objects
– White light is the presence of ALL the
colors of the visible spectrum.
– Black objects absorb ALL the colors and
no light is reflected back.
© 2000 Microsoft Clip Gallery
16.2 How we see other
colors
• The additive primary colors
are red, green, and blue.
• We don’t see everything
white because the strength
of the signal matters.
• All the different shades of
color we can see are made
by changing the proportions
of red, green, and blue.
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16.2 How we see the
color of things
• Colored fabrics and paints
get color from a subtractive
process.
• Chemicals, known as
pigments, in the dyes and
paints absorb some colors
and allow the color you
actually see to be reflected.
• Magenta, yellow, and cyan
are the three subtractive
primary colors.
16.2 Color and Vision
• When all the colors of the rainbow are
combined, we do not see any particular color.
• We see light without any color.
• We call this combination of all the colors of light
"white light".
16.2 Color and Vision
Key Question:
How do we see
color?
*Students read Section 16.2
AFTER Investigation 16.2
16.2 How we see the
color of things
When we see an object, the
light that reaches our
eyes can come from two
different processes:
1. The light can be emitted
directly from the object,
like a light bulb or glow
stick.
2. The light can come from
somewhere else, like the
sun, and we see the
objects by reflected light.
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16.2 Why are
plants green?
• Plants absorb energy
from light and convert
it to chemical energy
in the form of sugar
(food for the plant).
• Chlorophyll is an
important molecule
that absorbs blue and
red light.
How the Eye Works Retina
Correcting Vision LASIK Surgery
The process:
• After your eye has been numbed with "eye drop" anesthesia, an instrument known as an eyelid speculum will be positioned to hold your eyelids open. You will remain awake and comfortable throughout the procedure.
• A small suction ring will be placed around the cornea and serves as a platform for the microkeratome.
• The microkeratome separates the surface layers of the cornea, and the corneal flap is folded back.
• You will be asked to look at a target light while the Excimer laser reshapes the corneal tissue. A clicking sound can be heard as each microscopic layer of tissue is vaporized. This process will last from seconds to minutes, depending on the amount of correction necessary.
• The corneal flap is then placed back into its original position and allowed to dry for a few minutes.
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How the human eye
sees color
• The retina in the back of
the eye contains
photoreceptors.
• These receptors release
chemical signals.
• Chemical signals travel
to the brain along the
optic nerve.
optic nerve
Photoreceptors in the Eye
• Cones respond to
three colors: red,
green and blue.
• Rods detect
intensity of light:
black, white,
shades of gray.
Brain interprets Fooling your brain
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http://www.michaelbach.de/ot/
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Fooling your brain
Mirrors
• Mirror: a plane sheet of glass or
similar material with a smooth, shiny
coating on one side.
Types of mirrors
• Plane mirror: a flat mirror that usually
shows a “correct” virtual image.
• A virtual image is an upright image
whose left and right sides are
reversed.
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Types of mirrors
• Concave mirrors: a mirror with a
surface that curves inward, thinner in
the middle.
• Concave mirrors can
form virtual images or
a real image depending
on the bend of the mirror.
Types of mirrors
• Concave mirrors rely on the focal
point in order to produce images.
• Focal point: the point in which light
rays meet to form a picture.
Types of mirrors
• Convex mirrors: a mirror with a surface that curves outward, thicker in the middle.
• Convex mirrors form virtual images that are always smaller than the originals.
• Convex mirrors allow you to see a larger area.
Lenses
• Lens: a curved piece of glass or
other transparent material that
refracts light.
• A lens forms an image by refracting
light rays that pass through it.
Convex Lens
• Convex lens: thicker in the center than at the edges that focuses rays of light (magnify).
• Convex lenses magnify
things or make them
look bigger.
• Examples are
microscopes,
magnifying glasses,
and telescopes.
Convex Lens
• Used in things like microscopes,
magnifying lenses, and in some
telescopes.
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• Concave lenses: thinner in the center and
thicker at the edges (minimize).
• Concave lenses form
virtual images that are
usually smaller than
the original.
• Used in eyeglasses
to correct for nearsightedness.
Concave Lens Concave lens
• Mainly used to correct vision in eye
glasses and contact lenses.
16.2 How does a color
TV work?
• Televisions give off light.
• To make color with a TV, you can use red, green, and
blue (RGB) directly.
• The screen is made of tiny
red, green, and blue dots.
• The dots are called pixels
and each pixel gives off its
own light.
• TV sets can mix the three
colors to get millions of
different colors.
Uses
• Lasers- produce very bright beams of
light.
– The atoms all give out their light together
Uses of Laser (1)
• In medicine
– to break up gallstones and kidney stones,
– to weld broken tissue (e.g. detached retina)
– to destroy cancerous and precancerous cells;
at the same time, the heat seal off capillaries,
– to remove plaque clogging human arteries.
Uses of Laser (2)
• In industry
– to drill tiny holes in
hard materials,
– for welding and
machining,
– for lining up
equipment precisely,
especially in
inaccessible places.
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Uses of laser (3)
• In everyday life
– to be used as bar-code
readers,
– to be used in compact disc
players,
– to produce short pulses of
light used in digital
communications,
– to produce holograms.
CDs
• CDs use laser light
– and reflection-
– The surface has tiny pits and smooth bits in a spiral pattern
– the pits scatter light from a laser
– the smooth bits reflect the light
– so a coded message is read.
spiral
Holography
• Holography is the production of holograms by the use of laser.
• A hologram is a 3D image recorded in a special photographic plate.
• The image appears to float in space and to move when the viewer moves.
Holograms
How holograms work Laser show
• Physics is in everything
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Total Internal Reflection
• Involves both reflection and refraction
• When light bends (refracts) enough it is
reflected rather than transmitted.
Total Internal Reflection
• Fiber Optics
– Transmit signals as
flashes of light.
– Since light has a higher
frequency than radio
waves, much more
information can be
carried on a light beam
than on a radio waves.
16.1 Light carries
information
• The fiber-optic networks you read
about are pipelines for information
carried by light.
16.1 Light carries
information
• In some cities, a
fiber-optic cable
comes directly
into homes and
apartments
carrying
telephone,
television, and
Internet signals.
• The process of making light with heat
is called incandescence.
• Incandescent bulbs generate light
when electricity passes through a thin
piece of metal wire called a filament.
16.1 Electric Light
• The filament heats up
and gives off light.
• The other common kind of electric light is the
fluorescent bulb.
16.1 Electric Light
• Fluorescent bulbs
convert electricity
directly to light without
generating a lot of
heat.
• Fluorescent bulbs use
high-voltage
electricity to energize
atoms of gas that fill
the bulb.
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16.1 Light intensity
• The intensity of light from a small source follows
an inverse square law because its intensity
diminishes as the square of the distance.
16.1 The speed of light
• The speed at which light travels through
air is approximately 300 million meters
per second.
• Light travels almost a million times
faster than sound.
16.1 The speed of light
• The speed of light is so important in
physics that it is given its own symbol,
a lower case c.
• The best accepted experimental
measurement for the speed of light in
air is 299,792,500 m/sec.
• For most purposes, we do not need to
be this accurate and may use a value
for c of 3 × 108 m/sec.
16.1 Reflection and
refraction
• Another example of
refraction of light is the
twinkling of a star in the
night sky
• As starlight travels from
space into the Earth’s
atmosphere, the rays are
refracted.
• Since the atmosphere is
constantly changing, the
amount of refraction also
changes.
16.3 Photons and Atoms
• Key Question:
How does light fit
into the atomic
theory of matter?
*Students read Section 16.3
AFTER Investigation 16.3
16.2 Photons and
intensity
• Intensity measures power per unit area.
• There are two ways to make light of high intensity.
– One way is to have high- energy photons.
– A second way is to have a lot of photons even if they
are low-energy.
The number and
energy of photons
determine the
intensity of the light.