2. Electromagnetic Radiation
• We know that a changing magnetic field can
cause the creation of an electric field.
– Electromagnetic Induction
• A changing electric field may also cause the
creation of a magnetic field.
• The result? The form of radiation we think of as
both light and radio.
3. Electromagnetic Waves
• Electromagnetic waves
consist of vibrating
electric and magnetic
fields.
• All EM waves move at
the speed of light (in a
vacuum).
– velocity = frequency *
wavelength still holds, but
the velocity is now the
speed of light.
5. Example Problems
• Is it correct to say that a radio wave is a low-
frequency light wave? Is a radio wave also a
sound wave?
• What is the principal difference between a
radio wave and light? Between light and an x-
ray?
6. Matter and EM Waves
• A vibrating electric
charge creates an EM
wave.
• An EM wave can cause
a charge to vibrate.
• This is similar to how a
vibrating tuning fork can
cause another tuning
fork to vibrate.
• In changing the way it
vibrates, a material may
absorb or emit EM
waves.
7. Transparency
• When light is absorbed and quickly re-emitted an
object may appear transparent.
– Reemission occurs before the energy can be changed to
heat through collisions.
8. Color
• White light is made up of
many colors (together they
appear white).
• Most objects do not emit
light – they reflect it.
• An object will appear to be
the complement of the colors
it absorbs.
• Absorbing all colors = black.
Absorbing none (reflecting
all) = white.
• The color something
appears depends on the
lighting.
9. Human Eye
• Rods (color insensitive)
and cones (color
sensitive).
• Diagram in lower right
shows the sensitivity of
the 3 cone types.
• Sometimes one of these
cone types is missing or
mutated.
11. Color Mixing
• By mixing three colors of
light (red, green and blue)
we may produce most any
color we can see.
• Red + Green + Blue = white.
• Red + Blue = Magenta
• Red + Green = Yellow
• Blue + Green = Cyan
12.
13. Pigments
• If you mix red, green and blue pigments you
get an ugly dark brown mess.
• Mixing of paints and dyes is different from
mixing of colored light.
• Pigments work by color subtraction:
white – cyan = red
white – yellow = blue
• We tend to use Cyan, Magenta, Yellow + Black
for printing.
14. Why is the sky blue?
• Scattered light.
– Absorbed light is re-
emitted in all directions.
– High frequency light
(blue) is more strongly
scattered than low
frequency (red).
15. Why sunsets are red
• Blue light is scattered
away (subtracted) from
the white light.
• When the path length is
very long (sunrise,
sunset) this subtraction
becomes visible.
16. Why clouds are white
• Different sized particles tend to scatter different
color light:
– small particles scatter blue
– medium particles scatter green
– larger particles scatter red
• Together we get white.
17. Polarization
• Light is a transverse
wave.
• Transverse waves can
have an orientation.
– In the picture to the left,
the top wave is oriented
up/down.
– The bottom wave is
oriented left/right.
18. • A polarizer has the ability to
block waves with their
electric fields oriented in
particular directions.
• When two polarizers are
aligned, they pass light.
• When they are crossed they
block light.
– LCD monitors work using this
principle.
– Some 3d movies take
advantage of the effect.
– Also used in some sunglasses.
