Light Waves
Unlike sound, light does NOT need a medium
to propagate or move through.
Light travels the fastest through vacuum
space – whereas sound does not exist in a
vacuum!
The speed of light in vacuum space is
300,000,000 m/s (3.0 x 108 m/s).
Unlike sound, light is a transverse propagation
of energy.
Light is an electromagnetic wave. It has an
electric part and a magnetic part.
The Electromagnetic
Spectrum or EM spectrum
contains all of the frequencies or
wavelengths of visible and
invisible light .
Frequency vs. Wavelength
 Waves
with the longest wavelengths
(λ) have the lowest frequencies and
the lowest energy.
 Ex:
Radio, Microwave, Infrared
 Waves
with the shortest wavelengths
(λ) have the highest frequencies and
the highest energy.
 Ex:
Ultraviolet, X-Ray, Gamma
Selective Transmission
 Some
substances allow only some
frequencies of light to pass through;
also called partial transparency.
 Ex:
Blue glass allows blue light
frequencies to pass through but absorbs
all other frequencies and turns them
into heat. The glass warms up a little.
 Ex:
The earth’s atmosphere allows us to
send radio waves to operate spacecraft.
Reflection of Light
Light waves bouncing off a surface or at
the boundary of a medium.
Law of reflection:
Angle of incidence =
angle of reflection
Selective Reflection
 Things
reflect the colors they do
NOT absorb.
 The reflected color (frequency) is
visible to the human eye.
 Ex:
The leaves of plants have a chemical
called chlorophyll that absorbs all colors
(light energy), except the green
frequencies they reflect.
Visible Light = R O Y G B V
e r e
d a l
n l
g o
e w
r l
e u
e e
n
i
o
l
e
t
Only reflected colors are visible to the human eye.
Chlorophyll Light Absorption
Chlorophyll b
Chlorophyll a
V
B
G
Y O
R
Refraction of Light
 Light
refracts (bends) when it goes
from one medium into another, like from
air to water.
This pencil
appears broken
in water
because of the
refraction of
light.
When white light passes through a narrow slit and
then through a prism, each wavelength (λ) refracts
(or bends) at a different angle. Violet (the
shortest wave) bends the most and red (the
longest wave) bends the least, hence the rainbow.
Illusions caused by refraction
During sunsets, the sun appears
higher in the sky than it actually
is because of refraction.
 The
amount of bending depends on the
material’s index of refraction.
 Diamonds have a high index of
refraction (lots of bending), while glass
has a lower index of refraction (less
bending).
Wavelengths (λ) of visible light
Human eyes send a different signal to the brain for
each λ. The brain interprets each as a different color.
Polarization of Light
Light is polarized when all its planes of
travel are reduced to one single plane.
We can use this property of light to make
sunglasses or other polarized materials.
Doppler Effect:
 The
shift in frequency when the light
source and the observer are moving toward
or away from each other.
 Ex:
 Galaxies that are moving away from ours
appear red (redshift) , while those moving
toward ours appear blue (blueshift).
Doppler Effect:
Light Diffraction
A light beam going through a very narrow
opening spreads out as the opening gets smaller.
The light blurs and loses its brightness.

Light diffraction is a
problem to scientists
who study things with
a light microscope.
Lenses


A convex lens
focuses parallel
light rays onto
a focal point.
A concave lens
spreads
parallel light
rays away from
each other.
Compare:

Light and sound can be refracted.

Light and sound can be reflected.

Light and sound can be diffracted.

Light and sound have a Doppler effect.

Light and sound are both waves of energy.
Contrast:

Light can be polarized; sound cannot.

Light speed is almost a million times faster than
sound.

Light is a transverse wave; sound is longitudinal.

Light can go through a vacuum; sound cannot.

Light is electromagnetic; sound is compressional.