LIGHT
(AND THE ELECTROMAGNETIC SPECTRUM)
ELECTROMAGNETIC WAVES
Electromagnetic Radiation is just a name for the
range of radiation….(feel free to start singing)
 Light is both a particle and a wave
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Light particles are called photons (mass-less bundles of
concentrated electromagnetic energy)
We only see light in the range of 400-700 nm
speed of light = c = 3.0*108 m/s
c = λ*f
WHY ARE SOME THINGS SEE-THROUGH?
Light is energy carried in an electromagnetic wave
that is generated by vibrating electric charges.
 Visualize the electrons in an atom as connected by
imaginary springs.
 When light hits the electrons, they vibrate.
 The natural vibration frequencies of an electron
depend on how strongly it is attached to a nearby
nucleus.
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TRANSPARENCY
If frequency of the light ≠ natural frequency of the
material  electrons forced into vibration with
small amplitudes
 The atom holds the energy for less time 
less chance of collision with neighboring atoms 
less energy transferred as heat
 Energy of the vibrating electrons reemitted as
transmitted light
 Materials that transmit light are transparent.
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OPACITY
If frequency of the light = natural frequency of
the material  electrons forced into vibration
with large amplitudes
 The atom holds the energy for more time 
more chance of collision with neighboring
atoms 
more energy transferred as heat
 Materials that absorb light without reemission
and thus allow no light through them are
opaque.
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GLASS: AN EXAMPLE
Glass is _______________ to visible light.
 BUT…Electrons in glass have a natural vibration
frequency in the ultraviolet range
 UV light shines on glass 
resonance occurs 
e- forced into vibration with higher amplitudes 
atoms hold energy for more time 
collide with neighboring atoms 
energy LOST to heat
 UV light does not pass through glass
 Glass is ____________ to UV light.
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GLASS: EXAMPLE CONTINUED
When the EM wave has a lower frequency than
UV, as visible light does, e- are forced into
vibration with smaller amplitudes 
atom holds the energy for less time 
less chance of collision with neighboring atoms

less energy is transferred as heat
 Energy of the vibrating electrons is reemitted as
transmitted light.
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SPEED OF LIGHT IN MATERIALS
In a vacuum, the speed of light is a constant
3 x 108 m/s
 Atmosphere: very close to c
 Water: 0.75c
 Glass: 0.67c
 Diamond: 0.40c
 When light emerges from these materials into
the air, it travels at its original speed, c
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SHADOWS
A thin beam of light is called a ray
 When light shines on an object, some of the rays
may be stopped while others pass on in a straight
line path
 A shadow is formed where light rays cannot reach
 Fuzzy part around the edges of the shadow
happens when:
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Light from one source is blocked but where other light
fills in or
 Where light from a source is only partially blocked
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POLARIZATION
When light from a lamp or the sun shines on a
polarizing filter, the light that is transmitted is
polarized
 Light will pass through a pair of polarizing filters
when their polarization axes are aligned, but not
when they are crossed at right angles.
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3D
Hold an upright finger at arm’s length and see how
it switches position relative to the background as
you alternately close each eye.
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Vision in three dimensions depends on the fact that both
eyes give impressions simultaneously, each eye viewing a
scene from a slightly different angle.
The view seen by each eye is different.
The combination of views in the eye-brain system gives
depth.
A pair of photographs or movie frames, taken a short
distance apart (about average eye spacing), can be seen in
3-D when the left eye sees only the left view and the right
eye sees only the right view.
3D MOVIES!
Movies project the pair of views through
polarization filters onto a screen.
 Their polarization axes are at right angles to each
other, so the right eye sees only the right view and
the left eye sees only the left view.
 Overlapping pictures look blurry to the naked eye
 To see in 3-D, the viewer wears polarizing
eyeglasses with the lens axes also at right angles
 Each eye sees a separate picture, just as in real
life. The brain interprets the two pictures as a
single picture with a feeling of depth
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LIGHT PART II: COLOR
(AND THE ELECTROMAGNETIC SPECTRUM)
AND ON TO….COLOR: THE SPECTRUM
By passing a narrow beam of sunlight through a
triangular-shaped glass prism, Newton showed
that sunlight is composed of a mixture of all the
colors of the rainbow
 spectrum = spread of colors
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ROYGBV
white light = combo of all the colors
 black = absence of light
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WHY DO WE SEE DIFFERENT COLORS?
The color of an opaque object is the color of the
light it reflects
 The color of a transparent object is the color of the
light it transmits
 Most materials absorb light of some frequencies
and reflect the rest.

Material absorbs light of most visible frequencies &
reflects red  the material appears red
 Reflects light of all the visible frequencies  it will be
the same color as the light that shines on it
 Absorbs all the light that shines on it  it reflects none
and is black
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SUNLIGHT
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White light from the sun is a composite of all the visible
frequencies.
The brightness of solar frequencies is uneven
Lowest frequencies of sunlight in the red region
Not as bright as those in the middle-range yellow and
green region
Yellow-green light is the brightest part of sunlight.
COLORED LIGHT
Light of all the visible frequencies mixed
together produces white
 White also results from the combination of
only red, green, and blue light
 R, G, B called additive primary colors
 Red + Green light = Yellow
 Red + Blue light = Magenta
 Green + Blue light = Cyan
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COLORED PIGMENTS
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Red + green + blue paint  muddy dark brown
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Not white!
The mixing of paints and dyes is an entirely different process
from the mixing of colored light.
When paints or dyes are mixed, the mixture absorbs all the
frequencies each paint or dye in it absorbs
Example:
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Blue paint reflects mostly blue light, but also violet and green
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Yellow paint reflects mostly yellow light, but also red, orange, and
green
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It absorbs red, orange, and yellow light.
It absorbs blue and violet light.
When blue and yellow paints are mixed, between them they
absorb all the colors except green
FUN THINGS COLOR HELPS US UNDERSTAND
How do color television sets work?
 How is color printing done?
 Why is the sky blue?
 Why are sunsets red?
 Why is water greenish blue?
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ATOMIC SPECTRA
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Electrons surrounding the atomic nucleus have welldefined orbits
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Atom absorbs external energy  one or more of its
electrons is boosted to a higher energy level
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AKA – well defined energy levels—lower energy near the
atomic nucleus and higher energy farther from the nucleus
‘excited state’
Electron quickly drawn back to its original or a lower
level  atom emits a pulse of light called a photon
When made to emit light, every element has its own
characteristic color (the “fingerprints” of the elements)
The light is analyzed using a spectroscope
 The spectrum of an element appears not as a
continuous band of color but as a series of
lines called a line spectrum
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 Each
line corresponds to a frequency of light
Spectral lines seen in the spectroscope are
images of the slit through which the light
passes
 Much of the information that physicists have
about atomic structure is from the study of
atomic spectra
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