Map to Help Room (G2B90)
Lecture room
Help room
Homework
• Turn in your homework at the beginning of class
next lecture. It will be collected shortly after
lecture starts.
• Put your homework in the appropriate
alphabetized box on the right side of the back of
the lecture hall (the right as I would see it from
the front)
• Please consider coming to office hours before
emailing us with homework questions.
Clickers
• Register your clicker at iclicker.com
• No exceptions if you are absent, forget your
clicker, run out of batteries, etc.
• You do get more points for a correct answer, so
try to figure it out!
Chapter 1, continued
• Electromagnetic waves
– Is there a medium?
– What is “wiggling”?
– Electric and magnetic
fields
• EM Spectrum
–
–
–
–
Visible light
Radio waves
Microwaves
Xrays
• Resonance
– Why do we only see some
EM waves?
– Examples and demos
• Light sources
– Incandescent bulbs
– Fluorescent bulbs
– Other sources
White Light through a Prism
400 nm
700 nm
A prism spreads out the over- lapping wavelengths
in white light into different spatial locations where
they can be seen as colors.
Clicker Question
• Picture a rope with a pulse disturbance moving horizontally.
How does the distance and direction traveled by pieces of the
rope compare to the distance and direction traveled by the
pulse?
a) The rope piece travels more than the pulse, in the same
direction
b) The rope piece travels more than the pulse, at a right angle
to the pulse
c) The rope piece travels less than the pulse, in the same
direction
d) The rope piece travels less than the pulse, at a right angle to
the pulse
Clicker Question
• Which is the correct light ray to describe how we
“see” the apple? (the color of the arrow is not
relevant)
A
B
E
D
C
Remember the light has
to come from the bulb
first, so B is not correct.
C is the correct light ray
Electromagnetic (EM) Waves
• Light is part of the Electromagnetic Spectrum
• Unlike sound or rope waves, EM waves do not
need a material to transmit them, they can travel
through empty space
• So what is actually “wiggling” in an EM wave?
Electric and Magnetic Fields
• Static electricity
• Magnetic field lines
Electromagnetic Waves
• Made up of oscillating electric and magnetic fields
EM Waves: Key Points
• Waves can travel in empty space
• The oscillating fields are at right angles
(perpendicular) to the direction of wave
motion
Electromagnetic Spectrum
Electromagnetic Spectrum
Clicker Question
The speed of light is 3 x 108 m/s
A radio signal takes 2.5 x 10-3 s
(2.5 milliseconds) to travel from
Boston to DC. What is the
distance between these cities?
a) 1200 km
b) 750 km
c) 7500 km
d) 500 km
e) 1000 km
distance =speed * time
3 x 108 m/s * 2.5 x 10-3 s
7.5 x 105 m
750 km
Images at different frequencies
IR
Radar
UV
White light
Xray
Millimeter Wave Scanning
Waves and Resonance
• Oscillating systems have a special frequency at which
they like to vibrate, called the resonant frequency
• External driving of these systems at the resonant
frequency will result in the most efficient transfer of
energy
• Ex: Swings, rocking your car out of a snowbank,
washboard ripples on a dirt road, shattering a crystal
glass, dye bleaching, light perception in your eye,
washing machine
Example: Pendulum
• What happens when we change the length of the
string?
• What happens when I change how fast I move my
hand?
• The same process occurs in our eyes, which are not
sensitive to frequencies outside what we call
“visible light”
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
– 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
– EM waves with the wrong frequency do not excite the
resonance, and we don’t “see” them.
• Light can also be emitted by atoms, which is also a
form of resonance
– This is how fluorescent lights, lasers and LEDs work.
Incandescent Light Bulbs
Incandescent Light Bulbs
Filament with current of electrons
which hit into atoms causing light to
be emitted
Atom
Atom
Electrons
Atom
Light emitted at many
different resonance
frequencies of atoms
appears as white light
Incandescent Light Bulbs
• A continuous light source
• Almost 90% of its emission is
invisible to the human eye,
producing heat and wasting
energy
Clicker Question
The light from an ordinary incandescent light
bulb appears white because
a) A current of electrons excites a resonance in
atoms at the frequency of white light.
b) A current of electrons excites resonances in
atoms at many different frequencies
c) A current of electrons wiggles at the
frequency of white light
Fluorescent Light Bulbs
• Fluorescent bulbs have a lower current and power
usage for the same light output in the visible range
• How do they do this?
The atoms inside a fluorescent bulb have
ultraviolet resonant frequencies
Atom
Atom
Electrons
Atom
Invisible ultraviolet light
Phosphors
white light
Fluorescent Light Bulbs
• Because the phosphors emit at very specific resonant
frequencies, the spectrum is not continuous
Incandescent vs. Fluorescent
Neon lights
• Produced the same way, but with a different set of
atoms in the tube to produce the different colors.
Light Emitting Diodes (LEDs)
• A semiconductor system
• Charges are initially separated
• An applied current pushes
them “up the hill”, where they
can recombine and emit light
Clicker Question
• What kind of bulbs do you think they use to keep
food hot at a buffet bar?
a)
b)
c)
d)
Incandescent
Fluorescent
Neon
LEDs
Color Temperature
• Light from ideal sources is generally a mixture of
different wavelengths
– Think of the rainbow generated by sunlight
– The spectra of different light bulbs
• The mixture of wavelengths can be understood
by asking how bright is the mixture at each
wavelength
• How do these different formulations change the
color of the light?
The hotter the source the more bluish the white light.
The cooler the source the more reddish the white light
These are curves of the intensity of
each of the wavelengths present in
an "off-white" light as the (color)
temperature of the filament in the
light bulb increases.
Color Temperature in Fluorescent Lights
Fluorescent light bulbs all feel the same temperature
when you touch them, but the phosphors can be
selected to simulate a specific color temperature.
From www.usa.phillips.com
Here is how a picture changes under lighting with
different color temperatures
Higher
color temp
more bluish
You can also change the color
temperature in imaging
processing software
Lower
color temp
more reddish
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