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Light
What causes light?
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Objects that give off their
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own light are called luminous.
They are light sources.
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The sun is luminous. The
moon is not.
ALL light energy is produced
when electrons change
energy levels within an atom. •
When an electron absorbs
extra energy, it jumps to a
higher energy level.
When it drops back to a
lower energy level, a
packet of light energy is
given off.
A packet of light energy is
called a photon.
photon
photon
Models of Light - Wave or Particle?
Evidence: Light is a wave.
• Light has interference patterns
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and particles do not.
Light behaves like a transverse
wave when it is diffracted.
Particles cannot be diffracted.
Light waves can be polarized—
(made to vibrate in only one
direction). Particles cannot be
polarized.
Evidence: Light is a particle.
• Blue light produces electricity,
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but no amount of red light can.
If light were waves, then a whole
lot of red light should produce
electricity. The photoelectric
effect proves that light is a
particle.
Changing light intensity
(brightness) is best explained if
light is a particle, not a wave.
photon
Einstein’s conclusion: Light is both a wave and a particle!
Speed of Light
• In 1926, Albert Michelson first
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measured the speed of light
accurately.
The speed of light changes,
depending on the medium that
the light is traveling through.
• Light travels fastest through empty
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space--at a speed of about
300,000,000 m/sec !
Light travels through air almost as
fast as through empty space, but it
travels more slowly through liquids,
and even slower still through
solids.
It takes light 8.3 minutes to get from
the Sun to the Earth.
* A light-year is not a measure of time. It measures
the distance that light travels through space in one
year.
The Electromagnetic Spectrum - 1
• Light always has both an
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electric field and a magnetic
field traveling with it. This is
why light is called
electromagnetic radiation.
Light waves are also called
electromagnetic waves.
They are different than other
energy waves, because they do
not need a medium.
• All electromagnetic waves are not
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the same.
They can have different
wavelengths, different frequencies,
and carry different amounts of
energy.
When all light waves are arranged
in order according to their
wavelength and frequency, we call
this arrangement the
electromagnetic spectrum.
The Electromagnetic Spectrum
Long wavelength, low frequency
Short wavelength, high frequency
low energy photons
Radio Waves
Microwaves
Television
high energy photons
Visible Spectrum
Infrared
Ultraviolet
Gamma Rays
X-Rays
The Electromagnetic Spectrum - 2
• The visible spectrum is the part • The invisible spectrum is the part
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of the electromagnetic spectrum
that we can see.
The longest wavelength of
visible light is 750
nanometers,and is red light. A
nanometer is one millionth of a
millimeter.
The shortest wavelength of
visible light is 430 nanometers
and is violet light.
The colors of the visible
spectrum in order from longest
to shortest wavelength are: red,
orange, yellow, green, blue,
indigo, and violet. (ROY G. BIV)
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of the electromagnetic spectrum
that we cannot see.
Radio Waves - have the longest
wavelengths.
Microwaves - High energy radio
waves are called microwaves.
Infrared Waves - have
wavelengths slightly longer than
visible red light. This type of
radiation makes molecules move
faster, causing heat!
Ultraviolet Waves - the main
source of UV radiation is the sun.
UV light kills bacteria, makes your
body produce vitamin D, and
causes skin cancer and sunburn.
X-Rays - high energy light that can
travel through body cells.
Gamma Rays - highest energy
light waves are used in radiation
treatments for cancer.
Interactions of Light: 1
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When light hits an object, the light
can behave in several ways: It can
be reflected, or absorbed, or pass
through the object.
Objects that are black absorb all of
the light that hits them.
Objects that are white reflect all of
the light that hits them.
A red object absorbs all other
colors and reflects back only red.
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Objects that let
almost all of the
light pass through
are called
transparent.
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Objects that
don’t let any
light pass
through are
called opaque.
Objects that let
some light pass
through but
scatter it, are
called translucent.
Interactions of Light: 2
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Light waves interact with the
surface of an object or with a
medium just like other waves do.
Light waves can be reflected,
diffracted, refracted, or
experience constructive or
destructive interference.
Light is reflected by a mirror.
Light is refracted when it
changes from one medium to
another.
• Constructive interference makes a
light brighter, but destructive
interference makes darkness.
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Interference produces bands of
light and dark.
• All animals do not see in color. • The three primary colors of
Dogs, cats, cattle and horses
cannot see colors, but apes,
humans and birds can.
light are red, green, and blue.
When all of these are mixed
together, they make white
light.
• Human eyes have two kinds of
receptor cells in the retina:
rods which see objects even in
dim light but in black and
white, and cones which see
colors.
• Paint is different from light. It is
made of pigment. The three colors of
pigment are magenta, yellow, and
cyan. When these are mixed together,
they make black pigment.
Light Sources
• Objects that produce their
own light are called
luminous.
• Objects that reflect light that
comes
from another source are called
illuminated.
• Light bulbs are luminous. Some
common kinds of light bulbs are
neon, fluorescent, incandescent,
tungsten-halogen, and sodiumvapor lights.
• Even some living things can
be luminous--lightning bugs
are an example of this process
of bioluminescence.
Vision - Parts of the Eye
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Iris and Pupil: The iris adjusts the size
of the pupil to control how much light
enters your eye.
Cornea and Lens: The transparent
cornea lets light into your eye. The
lens changes shape to focus the light
rays on your retina.
Sclera
Cornea
Retina
Pupil
Lens
Iris
Lens Muscles
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Retina: The retina is like a movie
screen. Light forms an image on
the retina. The rod and cone cells
are stimulated by the image.
Optic Nerve: Nerve fibers
connected to the rods and cones
join together to form the optic
nerve, which carries nerve signals
from your eyes to your brain.
Optic nerve
Reflection and Mirrors
• A reflection happens any time •
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light bounces off of an object.
Anything you can see reflects •
light to your eye.
A mirror is a flat or a curved
piece of glass that has been
painted on the back to give
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the maximum reflection.
Images in a mirror are
reversed from left to right.
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Object
Plane Mirror
Image
Mirrors are either plane, convex, or
concave.
A plane mirror makes an image that
seems to be behind the mirror. It is
also the same size as the object being
reflected. This is a virtual image,
which can only be seen in a mirror.
A concave mirror makes a real image,
but the distance between the object
and the mirror determines the kind of
image reflected.
A convex mirror forms a virtual image
that is larger than the object reflected.
The image is always upright.
Convex mirror
Concave mirror
Refraction and Lenses
• Refraction is the bending of light rays. Light usually travels in
straight lines and at a constant speed unless it changes from one
medium to another.
• A lens is a transparent material that refracts light. Lenses usually
have one or more curved surfaces. There are two kinds of lenses:
convex and concave.
• A convex lens is thin at the edges and • A concave lens is thin in the middle
and thick around the edges.
thick in the center. The image it forms
Concave lenses form images that
depends on how far the object is from
are upright and smaller that the real
the lens. A convex lens is used in a
object.
microscope or a camera. The lens in
your eye is convex.
Image
Object
Object
Convex lens
Image
Concave lens