Physics 1230: Light and Color
•  What is "science" and why
is it hard?
•  Why is learning science
like learning a foreign
language?
– Words have new and precise
meanings
–  Wave, image, ray, lens, white, exposure, file, see,
reflection, refraction, dispersion, particle
– Acquisition of a skill
– Can't master it without using
it
Course Business
2
How should I read the textbook?
Type of reading
Pages per hour
Newspaper or
Non-science non-
Physics 1230 textbook "Seeing
light novel
fiction textbook
the Light?
35 – 50 pgs per
15 – 30 pgs per h r 6 - 12 pgs per hr.
hr. or more
Need for taking
No
Sometimes take
Always reread, think about, and
notes or
notes or use
then take notes in a dedicated
rereading?
marker.
notebook. Don't use marker to
help you memorize a lot of facts.
Emphasize concepts and ways of
figuring things out in your notes.
What will be tested in this class?
How can I get a good grade?
Ability to:
Memorize
"Understand" new
Deeper understanding: Be able to
new facts
concepts at a
figure out new things using concepts
and
minimal level
definitions
Percent of grade:
25%
25%
50% (only 15% uses math)
Survey to be answered by clickers
1.  What is your background
for light and color?
Choose the one which
best describes your
science background
a)  I have had no physics in high
school or college
b)  I have had physics in high
school but not in college
c)  I have taken a physics course
OR a psychology course at the
college level
d)  I have taken more than one
physics or psychology courses
(high school and/or college
level)
e)  I am a science major
Survey to be answered by clickers
2.  Why did you take this
course? Give the answer
which best describes your
reason.
a)  Because of requirements
by the university
b)  Because it was
recommended to me
c)  Because it looked easy
d)  Because it looked
interesting
e)  I don't know why Intro and Ch. 1
•  Teasers - What is light?
–  Bending, bouncing and
spreading light
–  Rays and Waves
–  Mixing colors using light
–  You don't see what you
think you see
–  Rats and Rays
•  Scientific notation
–  Powers of 10 video
–  Another version
7
Scientific notation and metric system
•  Powers of 10 give a
shorthand notation for
very large numbers.
103
• 
= 1000
•  102 = 100
•  10 1 = 10
•  100 = 1
•  Or very small numbers
10-1
• 
= 0.1
•  10-2 = 0.01
•  10-3 = 0.001
•  Scientists don't use feet or miles
to indicate distances
•  They use
–  meters (m)
•  1 meter = 39.4 inches
–  kilometers (km)
•  1 km = 1000 m = 0.625 mi
–  centimeters (cm)
•  1 cm = 10-2 m = 0.394 inches
–  millimeters (mm)
•  1 mm = 10-3 m
–  nanometers (nm) •  1 nm = 10-9 m
Clicker question
• 
The wavelength of green
light is around 500 nm.
How many wavelengths
of green light fit into one
cm (0.4 inches, or a
fingertip)?
a) 
b) 
c) 
d) 
e) 
20 thousand
50 thousand
Two million
Two billion
5 billion
•  wavelength = 500 nm = 5 x 102 x10-9 m = 5 x 10-7 m
•  Hence 1 m = wavelength/
(5 x 10-7) = 107/5
wavelengths =
2 million wavelengths
•  Since 1 cm is 1/100 of a
meter there are 2 x
106/100 = 20,000
wavelengths in a cm
Intro and Chapter 1 Continued
•  Light belongs to a family of
waves called electromagnetic
(EM) waves (Physics 2000)
–  Other waves: rope waves,
water waves, sound waves, etc.
•  Sometimes EM waves are
called EM radiation
–  Radio waves
–  Radar and similar waves
•  microwaves
•  cell phone waves
– 
– 
– 
– 
Infrared or heat waves
Ultra-violet (suntan) waves
X-rays
Gamma rays
•  EM waves are created and
destroyed by emission and
absorption
–  Classical picture (Phys 2000)
•  wiggling electrons radiate radio
waves or radar waves
•  electrons in an atom are
resonant with emitted or
absorbed light waves or X-rays
–  Quantum picture (Phys 2000)
•  change of state of electrons in
atoms when bundles of wave
energy (photons) are emitted or
absorbed
•  Light sources
–  Incandescent light bulb
–  Neon light
–  Fluorescent light
Rays (a single beam of light, for example)
•  Single light ray
–  Ray from a laser acts like a
single light ray
–  Illustrate by laser light
through fog
–  Bounce off mirror
–  Bounce off white card
–  Put through water (bending)
•  We only see light when a
ray enters into our eye
–  Laser light is visible from
side because it is scattered
into our eyes
•  Rays from a flashlight
•  Rays from a light bulb
•  What about light coming
from everything in this
room? Two kinds of objects:
–  Self luminous objects (lights)
–  Objects which are not selfluminous are seen because of
light reflected off them
•  Turn out the light and we
don't see anything in the room
•  It's all reflected light with
many rays coming from
diffuse surfaces
Laser
Flashlight
Light
bulb
Light rays are invisible unless they enter
directly into our eye or are scattered by
smoke, fog or some object into your eye!
MANY reflected rays come
from all parts of Alex, including
his nose - a diffuse object
Incident ray from a light bulb
Bob sees Alex's nose because a reflected light ray enters Bob's eye!
Rays bend when they are directed at an angle
from air to water or glass
Air
•  This is the principle behind lenses
Glass or
water
Rays bounce when they reflect off a mirror or
shiny surface
•  This is called specular reflection.
•  How is it different from diffuse reflection?
Mirror
Waves
•  Rope waves
–  Created by oscillation of
my hand holding the rope
–  Finite speed of wave, but
rope segments do NOT
move in direction of wave
–  Rope segments move up
and down, not along wave
–  Note the change that occurs
when I oscillate my hand
faster
•  Radio wave transmitter
–  3 meters wavelength
–  (100 Mhz frequency)
•  Google search under
keyword "physics"
–  Water waves (circular
pattern)
–  Stadium waves
When rays come out in various different directions
from an object or objects, the wavefront is defined as
a curve or surface perpendicular to all the rays
Wavefront
Rays
Light
bulb
•  In this case the wavefront is
expanding out spherically from
the light bulb.
•  Wherever it intersects a ray the
wavefront is perpendicular to
that ray
–  More technically, the tangent
to the wavefront at the point of
intersection is perpendicular to
the ray •  The wavefront may be easier to
visualize than the rays
–  You throw a pebble into a
pond. The circularly
expanding water waves are the
wavefronts
Draw a wavefront for each of these sets of rays; how
can the rays be produced in each case? •  Produced by a laser, for example
•  Produced by two light bulbs,
for example
How are wave wiggles related to rays?
Light with a SINGLE
wavelength is called
monochromatic light
Speed of light in empty space is c = 186,000 miles/sec
= 3 x 108 meters/sec
Wavelength {
Amplitude {
Ray
Waveform
Note, the wave is NOT "red." I have colored it red.
We perceive it as red because of its wavelength. We see color when waves of different
wavelengths enter enter our eyes! Light with wavelength of 650 nm
appears red when it enters a viewers eye Light with wavelength of 520 nm
appears green when it enters a viewers eye Light with wavelength of 470 nm
appears blue when it enters a viewers eye The speed of light in empty space is the same for all wavelengths
Clicker questions
• 
• 
• 
Which of the light waves has
the longest wavelength?
Which of the light waves is
brightest?
Which of the light waves has
the highest speed in empty
space?
a)
b)
a)  b) c)
e)  They all have the same
speed
c)
Clicker question
• 
What does Alex see when
the wave at left with
wavelength 650 nm goes
by him?
a) 
b) 
c) 
d) 
e) 
Red
Blue
Green
White
Nothing
What happens when two or more waves with
different wavelengths reach your eye? Light with both wavelengths 650 nm and
520 nm appears yellow when it enters
a viewers eye Light with only wavelength 580 nm
ALSO appears yellow when it enters
a viewers eye (A DEEPER YELLOW
THAN FOR THE CASE ABOVE)
What is white light?
Light which is a mixture of 650,
520 and 470 nm wavelengths
(and possibly more wavelengths)
appears WHITE when it reaches
your eye
No single wavelength (mono-
chromatic) wave appears white
when it reaches your eye!
A prism
spreads out the
over- lapping
wavelengths in
white light into
different spatial
locations where
they can be
seen as colors. Period and frequency of a wave and
relation to wavelength and speed
• The period, T, is the time for
the wave to make one complete
cycle (say, top-bottom-top) AT
ONE FIXED SPOT Think of my hand moving to make the rope
wave. The period is the time for my hand
holding the rope to make one complete topbottom-top (or bottom-top-bottom) motion.
• The frequency, f, of the wave is equal
to one over the period: f = 1/T f has the units of 1/secs, which we
call Hertz (Hz)
• The frequency, f, is related to the
speed of the wave, c and its
wavelength, λ (lambda):
f λ = c
Why does f λ = c? The frequency (f) of light
× its wavelength (λ) = the speed of light (c)
•  If your car moved a distance of
20 miles in a time of 2 hours
what would be your (constant)
speed?
–  distance divided by time, or
•  v = 20/2 = 10 miles per hour
•  A light wave of a particular
wavelength moves a distance =
to its wavelength, l, in a time
equal to its period, T
–  Why?
•  The speed of the wave is the
distance the wave has moved
divided by the time to move that
distance (just like for the car)
•  speed of light = distance/time
•  c = l / T, or •  c = l times 1/T
  The frequency of the wave is f =
1/T, so the last equation becomes
•  c = λ times f or, more simply,
•  c = λ f
Since c is the same for all wavelengths, fλ= c gives one frequency,
f = c/λ for each monochromatic wave of wavelength λ. We can identify a wave by its frequency, f, OR by its wavelength, λ!
•  For example, consider red
light
–  Wavelength l = 650 nm =
650 x 10-9 meters
–  The speed of light is c = 3 x 108 meters/sec
–  The formula says fλ = c
–  Divide both sides by l to get
f = c/l. Hence,
–  f = (3 x 108)/(650 x 10-9) =
4.62 x 1014 Hz = the
frequency of red light
•  Or blue light:
–  Wavelength λ = 470 nm =
470 x 10-9 meters
–  The speed of light is STILL
c = 3 x 108 meters/sec
–  The formula says f λ = c
–  Divide both sides by l to get
f = c/λ. Hence,
–  f = (3 x 108)/(470 x 10-9) =
6.38 x 1014 Hz for blue light
•  Red has a longer wavelength
but smaller frequency than
blue! What is a resonance?
•  Many objects oscillate or vibrate
at special frequencies called resonant frequencies or resonances
•  When these objects are hit or
"shaken" by an external agent at a
frequency = to their resonant
frequency they will oscillate at
their resonant frequency.
–  Hand moving back and forth at
same frequency as pendulum’s
resonant frequency (or hit)
–  Tacoma straights bridge in the
wind
–  Car on a dirt road with regular
bumps (washboard effect)
•  The oscillations of the object are
largest when the "shaking" occurs
at the objects resonant frequency. –  We then say that a resonance has
occurred
–  Girl on swing being pushed by her
mother (mother’s push frequency
= swing frequency)
•  Energy is transfered from an
external agent to the object during
resonance.
–  Wineglass broken by an opera
singers voice
–  due to resonance between voice
sound frequency and natural
frequency of wineglass
Effect of resonance produced by military helicopter
blade going around at frequency resonant with the
helicopter body
What do resonances have to do with light?
•  When light is absorbed by
atoms we can think of this
as a resonance
•  When light is emitted by
atoms we can think of this
as a resonance
–  The light frequency may
match a certain frequency
of resonant vibration in the
atom. –  When this happens, the
energy of the light is
transferred to the atom and
the light disappears.
–  For example, we see light
rays of 470 nm coming into
our eyes because this light
excites a resonance in
certain atoms inside our
eyes
–  For example when an
electron hits an atom the
atom can gain energy in the
form of resonances. –  This energy in the atom can
then be released by another
resonant interaction in
which light is emitted and
the atom loses energy.
–  Each color of light emitted
corresponds to a particular
atomic resonance.
Physics 2000: Electromagnetic waves
http://www.colorado.edu/physics/2000
•  Demo of different
wavelengths
•  What is the electric force?
•  What are lines of electric
force field?
•  Wiggling electrons also
wiggles lines of electric
force field!
–  This generates
electromagnetic waves
–  Speed of light and relation
between frequency and
wavelength
•  In class demos
–  Magnets and lines of force
–  Transmitter
•  This picture of where EM waves
come from is technically correct
only for long wavelength EM
waves
•  More detailed picture
–  Lines of electric force field far
from a wiggling electron
–  Electric field arrows along a ray
–  Wiggling an electron near an ion
give loops of electric field lines
–  Radiation from an antenna
Which level of physics is needed to explain
what properties of light?
•  Image formation - –  ray theory
•  Wavelength color,
polarization and
diffraction - –  wave theory (electricity and
magneticsm)
•  Interaction of light with
atoms - –  quantum theory of photons
•  Constant speed of light no
matter how fast the source
or observer is moving - –  special theory of relativity
Light at
wavelengths
which we see
as colors are
part of a
wider family
of electro-
magnetic
waves
What happens when those other
electromagnetic waves enter our eyes?
Wavelength = 10 million nm
Note that the frequency is
f = c/λ
= (3 x 108 m/s)/(10-2 m)
= 3 x 1010 Hz
•  We don't see anything because
electromagnetic waves at this frequency cannot excite a
resonance in the atoms in our eyes.
•  Note that this wavelength is on
the order of a centimeter
•  These are microwaves just like
those sent out and received by your
cellphones (or used in a microwave
oven).
•  We cannot see them even
when they enter our eyes because
their wavelength is too long to
excite a resonance.
How does an ordinary incandescent
light bulb work?
Filament with current of electrons which hit
into atoms causing light to be emitted
Gas
Atom
Atom
Electrons
Electrode leading to the other side of
the wall plug
Atom
Light emitted at
many different
resonance
frequencies of atoms
appears as white
light
Electrode leading to one
side of the wall plug
How does a fluorescent bulb work?
•  A fluorescent bulb is more
efficient than an
incandescent bulb because
the current is smaller. The
bulb doesn't give out as
much heat energy.
–  Heat energy is undesirable in
a light bulb
–  Heat energy is invisible so it
doesn't bring us light
–  Heat energy is carried by
infra-red waves which we
cannot see.
•  An electron current is again
generated in a fluorescent
bulbs but the atoms hit by the
electrons now have resonances
at ultra-violet frequencies.
•  The ultra-violet rays emitted
by the atoms hit phosphors in
the inside coating of the
fluorescent tube. •  The phosphors emit visible
light which we see (demo)
•  The fluorescent bulb works at
lower currents with less heat!
The atoms inside a fluorescent bulb
have ultraviolet resonant frequencies
Atom
Atom
Electrons
Atom
Invisible ultraviolet light
Phosphors
white light
Neon lights have atoms with resonances at special colors
inside. They use alternating (AC) household current
(Demo)
Atom
Atom
Electrons
Atom
Neon light
It's a good idea to remember some rough
wavelengths associated with colors
•  Violet and blue are
what we see when
shorter wavelength
visible rays enter
our eyes.
–  They have
relatively higher
frequencies
•  Red is how we see
longer wavelength
visible rays
–  Red has a relatively
smaller frequency How does the light from a light bulb
depend on temperature?
•  Light from ideal sources is
generally a mixture of
different wavelengths
–  Think of the light from the
sun, which is broken up by
a prism
–  (Such light is called blackbody radiation) •  The mixture of
wavelengths can be
understood by asking how
bright is the mixture at
each wavelength
•  The result is a curve which
peaks at a certain wavelength
and falls off at higher or
lower wavelengths
–  The hotter the source, the
lower the wavelength at which
the peak brightness occurs
–  Demo using incandescent bulb
with controlled current (and
hence temperature)
–  This is important in moviemaking
The hotter the source the more bluish the white light.
The cooler the source the more reddish the white light
Color temperature of "white" can be understood by
mixing just three lights of different intensities Here is how a picture changes under lighting with
different color temperatures
Modern quantum mechanics replaces the general concept of
atomic resonances with the specific concept of a change in
discrete energy levels of electrons in the atom
•  Emission of light
•  Absorption of light
•  Atomic electron at high energy level
•  Atomic electron at a lower energy level
•  Atomic electron at a lower energy level
Light (a photon)
hits an atom
•  Atomic electron at high energy level (atom gains energy
from light)
•  Extra energy is carried
off by light (photon)
Physics 2000: Photons and the quantum picture
of how light interacts with matter
http://www.colorado.edu/physics/2000
•  Bohr atom
•  Transitions of electrons
between energy levels and
appearance or disappearance of photons