Electromagnetic waves consist of vibrating electric and magnetic fields that propagate at the speed of light. The electromagnetic spectrum includes visible light as well as other types of electromagnetic radiation such as radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays. Electromagnetic waves can be characterized by their wavelength or frequency, with radio waves having the longest wavelengths and lowest frequencies, and gamma rays having the shortest wavelengths and highest frequencies. Materials can absorb, transmit, or reflect electromagnetic waves depending on the wavelength and properties of the material.

1. Light

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.

4. Electromagnetic Spectrum

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.

10. Colorblindness

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

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.