Light
what is it?
Light
• what is it?
• wave or particle?
moving energy
Light
• what is it?
• wave or particle?
• how do we decide?
moving energy
Light
•
•
•
•
what is it?
moving energy
wave or particle?
how do we decide?
if a wave, what is waving?
(waving even in a vacuum?)
Light
•
•
•
•
what is it?
moving energy
wave or particle?
how do we decide?
if a wave, what is waving?
(waving even in a vacuum?)
Electric & Magnetic Fields
Properties of Light
•
•
•
•
•
•
•
•
speed of light
colors
reflection
shadows
refraction (bending)
energy theory
absorption of light
emission of light
Property 1: Speed of Light
• particle (photon) ?
no prediction
• wave (E&M) ?
in vacuum, v = c; in material, v < c
From experiment, we find that the wave
prediction works!
(Here c stands for the speed of light in vacuum,
which is 300,000,000 meters/second, or about 670
million miles per hour.)
Property 2: Color
• experiment ?
• particle (photon) ?
• wave (E&M) ?
Property 2: Color
experiment ?
visible order:
• red
• orange
• yellow
• green
• blue
• violet
Property 2: Color
experiment ?
invisible as well as visible
total spectrum order:
• radio
• microwave
• IR
• visible
• UV
• x-ray and gamma ray
Property 2: Color
particle (photon) ?
amount of energy per photon
determines “color”
Property 2: Color
particle (photon) ?
amount of energy
among different types:
x-ray - most energy;
radio - least
in visible portion:
violet - most energy;
red - least
Property 2: Color
• particle (photon) ?
• wave (E&M) ?
amount of energy
Property 2: Color
• particle (photon) ?
• wave (E&M) ?
amount of energy
frequency
among different types of “light”:
low frequency is radio (AM is 500-1500 KHz)
high frequency is x-ray & gamma ray
in visible spectrum:
red is lowest frequency (just above IR)
violet is highest frequency (just below UV)
Wavelength and Frequency
“Nice” sine waves have a simple relation for
wavelength and frequency: λ*f = v
where λ is the wavelength (distance from one
crest to the next one),
where f is the frequency (how many times one
location goes up and down a second),
and where v is the speed of the wave (how fast
the crest of the wave moves).
λ
v
Light
For light in vacuum, the speed of the light wave is
300,000,000 meters/sec, or about 670 million
miles/hour. We use the symbol “c” to denote this
value. Therefore for light in vacuum, we have:
λ*f = c .
Example: for a radio wave of frequency 100 MHz,
the wavelength is:
λ * (100 * 1,000,000 Hz) = 300,000,000 m/s, or
λ = 300,000,000 m/s / 100,000,000 Hz = 3 meters.
Nanometers
The wavelength of visible light is in the range
of 0.000000400 meters to .000000700
meters. This is an awkward way to write
these numbers. In Scientific Notation, this
becomes 4 x 10-7 m to 7 x 10-7 m. This is
still somewhat awkward, so we often use
the unit of nanometers (nm) which is 10-9
m; this gives the range for the wavelengths
of visible light to be 400 nm to 700 nm.
Colors: frequencies &
wavelengths (in vacuum)
AM radio
 1 MHz
100’s of m
FM radio
 100 MHz
m’s
microwave
 10 GHz
cm - mm
Infrared (IR)
1012 - 4x1014Hz mm - 700 nm
visible
4x1014 - 7.5x1014 700nm -400nm
Ultraviolet (UV) 7.5x1014 - 1017 400 nm - 1 nm
x-ray &  ray
> 1017 Hz
< 1 nm
Property 3: Reflection
• particle (photon) ?
• wave (E&M) ?
Property 3: Reflection
• particle (photon) ?
bounces “nicely”
• wave (E&M) ?
bounces “nicely”
bounces nicely means:
angle incident = angle reflected
Property 4:Light and Shadows
Consider what we would expect from
particle theory: sharp shadows
dark
light
dark
Light and Shadows
Consider what we would expect from
wave theory: shadows NOT sharp
crest
crest
crest
dark
dim light dim
dark
Light and Shadows
What DOES happen?
Look at a very bright laser beam
going through a vertical slit.
(A laser has one frequency
unlike white light.)
Diffraction: single slit
How can we explain the pattern from light
going through a single slit?
screen
w
x
L
Diffraction: single slit
In fact, we can break the beam up into 2n
pieces since pieces will cancel in pairs.
This leads to: (w/2n) sin(n) = /2 ,
or w sin(n) = n for MINIMUM.
screen
x
w
L
Diffraction: circular opening
If instead of a single SLIT, we have a
CIRCULAR opening, the change in
geometry makes:
the single slit pattern into a series of rings;
and
the formula to be: 1.22 n = D sin(n)
Diffraction: circular opening
Since the light seems to act like a wave and
spreads out behind a circular opening, and
since the eye (and a camera and a telescope
and a microscope, etc.) has a circular
opening, the light from two closely spaced
objects will tend to overlap. This will
hamper our ability to resolve the light (that
is, it will hamper our ability to see clearly).
Rayleigh Criterion: a picture
The lens will focus the light to a fuzzy DOT
rather than a true point.
D lens
Rayleigh Criterion: a picture
If a second point of light makes an angle of
limit with the first point, then it can just be
resolved.
lens
D
x
x’
s
s’
Limits on Resolution:
• Imperfections in the eye (correctable with
glasses)
• Rayleigh Criterion due to wavelength of
visible light
• Graininess of retinal cells
Limits on Resolution: further
examples
• hawk eyes and owl eyes
• cameras:
– lenses (focal lengths, diameters)
– films (speed and graininess)
– shutter speeds and f-stops
• Amt of light  D2 t
• f-stop = f/D
– f-stops & resolution: resolution depends on D
Limits on Resolution: further
examples
• other types of light
– x-ray diffraction (use atoms as slits)
– IR
– radio & microwave
• surface must be smooth on order of 