# Light Waves - PowerPoint

Document Sample

```					            Light Waves
• What we call light is a small portion of
the electromagnetic spectrum
• All the different colors are
electromagnetic waves with different
wave lengths
• Wave speed = frequency x wavelength
• EM radiation is alternating electric and
magnetic fields
Electromagnetic Waves
• Remember our field induction rules
• Changing magnetic/electric field
induces electric/magnetic field
Electromagnetic Spectrum
• Remember, the lower the frequency the
longer the wavelength and vice versa
• Wave speed of light is 300,000,000 m/s
Colors
• Lowest frequencies of visible light are
red
• Highest frequencies are violet
• The order is red, orange, yellow, green,
blue, violet
Producing EM Waves
• Need to vibrate charged particles
• We shake electrons in an antenna to
• We shake electrons in a magnetron to
make microwaves
• We shake electrons in atoms to make
light (this is a bit of a simplification
which we will correct later this semester
Light and Materials
• Electromagnetic Waves carry energy
• When light strikes a material, it vibrates
the lightweight electrons
Light and Materials
Light and Materials
• If the natural vibration frequency of the
electrons is close to the frequency of
the incoming light wave, the electron will
be set into big vibrations (resonance)
• It will absorb some energy from the EM
wave and will vibrate with larger
amplitude
• It can smack into the electrons in
neighboring atoms and vibrate them too
Light and Materials
• This cuts down on the intensity of the
light
• If all the light is absorbed by shaking
electrons we say the material is opaque
• If the light frequency doesn’t come close
to the natural frequencies of the
electrons, they shake a bit, but just re-
emit the energy
• Materials that can’t hold the energy are
Light and Materials
• There is a delay in transparent materials
between the time the electrons start
shaking and then re-emit the energy
• This means that the light travels more
slowly in these materials!!!
• We will use this fact later to do
interesting things with lenses
Light and Materials

See why the light travels more
slowly in a transparent material than
in a vacuum?
Light and Materials
• Infrared EM waves shake entire atoms
or molecules in materials
• Microwave EM waves effect the rotation
+
Remember our discussion of
water and how it has a
positive end by the hydrogen
atoms and a negative end by
-
the oxygen
Light and Materials
• Things that are opaque absorb the
energy carried by the light wave
• This means that they heat up
• Metals are opaque and shiny
• The free electrons close to the surface
shake and re-emit the energy back out
of the material
• They are reflectors!!!
Light and Air
• Our atmosphere is transparent to visible
light, some ultraviolet light and some
infrared light
• The ozone layer absorbs a lot of
ultraviolet light
• The ozone molecule is O3 and breaks
into a regular oxygen molecule O2 and
an oxygen atom
Colors
• Colored objects are a result of selective
reflection and absorption of light
• Sunlight is a mixture of colors
• We call this mixture white light
• When white light strikes a red object,
the red frequencies are reflected and
the other frequencies are absorbed
Sunlight
Colors
• White objects reflect all frequencies
• Black objects absorb all frequencies
• Objects can only reflect the colors of
light that shine on them
• If you shine blue light on a red object it
will look black
Colors
• We can also consider shining light
through semi-transparent objects
• Filters used on the lights in a theater are
materials that absorb some frequencies
and let other frequencies pass through
• Red filters transmit red light through the
material while other frequencies are
absorbed
Mixing Colors
• Our eyes and brains are very
complicated color processing sensors
and computers
• See the discussion on page 289 in the
text
• Artists have learned how to mix colors
to make pleasing combinations and to
create a huge array of different colors
• All based on what the sensors in our
Visual Response
Visual Response
Red + Blue = Magenta
Red + Green = Yellow
Blue + Green = Cyan

The way your
computer screen
makes colors. And
same thing.
Visual Response
Red + Blue = Magenta
Red + Green = Yellow
Blue + Green = Cyan

Magenta + Green =
White
Cyan + Red = White
Yellow + Blue = White
Selective Scattering
• This is why the sky is blue!
• The higher frequencies are scattered
more by the molecules in the
atmosphere
• Reds and oranges just pass through, so
the sky appears blue
Selective Scattering

Sunsets
are red!

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 90 posted: 2/19/2010 language: English pages: 24