Transcript Light

What is Physics?
First of all, Physics is a Science. So our first
question should be: What is a Science?
Science
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What is a science?
Physics is a science. Biology is a science.
Is Psychology a science?
Is Political Science a science?
Is English a science?
What makes a field of inquiry into a
science?
Scientific Method
What makes a field of inquiry into a science?
• Any field that employs the scientific
method can be called a science.
• So what is the Scientific Method?
• What are the “steps” to this “method”?
Scientific Method
• 1. Define the “problem”: what are you
studying?
• 2. Gather information (data).
• 3. Hypothesize (try to make “sense” of the
data by trying to guess why it works or what
law it seems to obey). This hypothesis
should suggest how other things should
work. So this leads to the need to:
• 4. TEST, but this is really gathering more
information (really, back to step 2).
Scientific Method
Note one thing about step 3: the predictive
power of the hypothesis gives us something
else to look for. We are in essence trying
to extend our common sense to areas in
which we initially have little common
sense.
Scientific Method
Fascinating Question
Is the scientific method really a never ending
loop, or do we ever reach “THE TRUTH”?
Scientific Method
Is the scientific method really a never ending
loop, or do we ever reach “THE TRUTH”?
Consider: can we “observe” or “measure”
perfectly? If not, then since observations
are not perfect, can we perfectly test our
theories? If not, can we ever be
“CERTAIN” that we’ve reached the whole
“TRUTH” ?
Scientific Method
If we can’t get to “THE TRUTH”, then why
do it at all?
We can make better and better observations,
so we should be able to know that we are
getting closer and closer to “THE TRUTH”.
Is it possible to get “close enough”?
Look at our applications (engineering): is
our current understanding “good enough” to
make air conditioners?
Physics
Now Physics is a science, but so are
Chemistry and Biology.
How does Physics differ from these others?
It differs in the first step of the method: what
it studies. Physics tries to find out how
things work at the most basic level. This
entails looking at: space, time, motion (how
location in space changes with time), forces
(causes of motion), and the concept of
energy.
Scientific Method and Light
To try to show the scientific method in action,
we’ll look at light.
Light
What is it?
Light
• What is it?
Moving energy
• There are two basic ways that energy can
move from one place to another: particles
can carry the energy, or the energy can
propogate in waves.
• Can light be explained as a wave or as a
particle?
Light
• What is it?
Moving energy
• Wave or particle? How do we decide?
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
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speed of light
colors
reflection
refraction (bending)
shadows
energy theory
absorption of light
emission of light
Property 1: Speed of Light
• particle (photon) prediction?
Property 1: Speed of Light
• particle (photon) ?
no prediction
• wave (E&M) prediction?
Property 1: Speed of Light
• particle (photon) ?
no prediction
• wave (E&M) ?
For a wave on a string, we can start from
Newton’s Second Law and get a wave
equation that leads to the relation:
vphase = [T/] (speed of wave depends on
parameters of the string the wave travels on
- T is tension in the string and  is the mass
density of the string)
Property 1: Speed of Light
• particle (photon) ?
no prediction
• wave (E&M) ?
Maxwell’s Eqs.
In a similar way to the wave on a string, we
can get a wave equation from Maxwell’s
Eqs for Electromagnetism. This predicts:
vphase = [1/oo]
where the o and o are the electric and
magnetic properties of vacuum.
Property 1: Speed of Light
• particle (photon) ?
no prediction
• wave (E&M) ?
Maxwell’s Eqs.
in vacuum:
v = [1 / {o o}]1/2 where
o = 1/{4k} = 1 / {4 * 9x109 Nt-m2/Coul2}
o = 4 * 1x10-7 T-s /Coul
v = [4*9x109 / 4*1x10-7 ]1/2 = 3 x 108 m/s = c
Property 1: Speed of Light
• particle (photon) ?
no prediction
• wave (E&M) ?
Maxwell’s Eqs.
in material,
vphase = [1/oo]
 = Ko , where K>1; and   o ; so
v<c
According to the wave theory, light should
move slower in material than in vacuum.
Property 1: Speed of Light
• particle (photon) ?
no prediction
• wave (E&M) ?
in vacuum, v = c; in material, v < c
we’ll come back to this when we look at
refraction.
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)
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
[This slide will be repeated after we see how we get these
values.]
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: Refraction
experiment ?
particle (photon)?
wave (E&M) ?
Property 4: Refraction
• experiment: objects in water seem closer
than they really are when viewed from air
eye
air
water
apparent
location
real object
Property 4: Refraction
• particle (photon) ?
incident ray
air
surface
water
refracted ray
Property 4: Refraction
• particle (photon) ?
incident ray
air
vxi = vxr
vyi < vyr
vxi
vyi
surface
water
therefore
vi < vr
vxr
vyr
refracted ray
Property 4: Refraction
normal line
• wave (E&M) ?
incident wave
air
surface
surfac
e
water
refracted wave
normal line
Property 4: Refraction
• wave (E&M) ?
incident wave
crest of following wave
crest of wave
crest of preceding wave
air
a
a
x
surface
w
water
w
refracted wave
normal line
Property 4: Refraction
• wave (E&M) ?
crest of wave
incident wave
 +  = 90o
 +  = 90o
crest of preceding wave
air
a
x
surface
w
sin() = a /x
sin() = w /x
water
refracted wave
normal line
Property 4: Refraction
• wave (E&M) ?
Snell’s Law
sin(a) = a/x and sin(w) = w/x
eliminate x: a/sin(a) = w/sin(w)
and use: f = v (or  = v/f) to get
f sin(a) / va = f sin(w) / vw
NOTE: since w < a, need vw < va
which is opposite to the prediction of the
particle theory but agrees with wave
prediction of Property 1 on speed!
Property 4: Refraction
• wave (E&M) ?
Snell’s Law
nicer form for Snell’s Law:
f sin(a) / va = f sin(w) / vw
Multiply thru by c/f to get
(c/va) sin(a) = (c/vw) sin(w)
and use definition of index of refraction:
n = c/v to get
na sin(a) = nw sin(w) Snell’s Law
Property 4: Refraction
• particle (photon) theory: vw > va
• wave (E&M) theory:
vw < va
• experiment ?
Property 4: Refraction
• particle (photon) theory: vw > va
• wave (E&M) theory:
vw < va
• experiment:
vw < va
particle theory fails!
wave theory works!
Property 4: Refraction
Snell’s Law: n1 sin(1) = n2 sin(2)
• NOTE: If n1 > n2 (v1< v2), THEN 1 < 2.
• NOTE: All 2 values (angles in the faster
medium) between 0 & 90 degrees work
fine.
• NOTE: Not all values of 1 (angles in the
slower medium) work!
Example: If n1 = 1.33, n2 = 1, and 1 = 75o, then
2 = inv sin [n1 sin(1) / n2] = inv sin [1.28] =
ERROR
Property 4: Refraction
Snell’s Law: n1 sin(1) = n2 sin(2)
If n1 sin(1) / n2 > 1 THEN there is NO
value of 2 that can satisfy Snell’s law
(unless you count imaginary angles!).
The math is trying to tell us that there is
NO transmitted ray. This is called
TOTAL INTERNAL REFLECTION.
Refraction and Thin Lenses
Can use refraction to try to control rays of
light to go where we want them to go.
Let’s see if we can FOCUS light.
Refraction and Thin Lenses
What kind of shape do we need to focus light
from a point source to a point?
lens with some shape for front & back
point
source
of light
s’ = image distance
s = object distance
screen
Refraction and Thin Lenses
Let’s try a simple (easy to make) shape:
SPHERICAL.
Play with the lens that is handed out
Does it act like a magnifying glass?
Refraction and Thin Lenses
Let’s try a simple (easy to make) shape:
SPHERICAL.
Play with the lens that is handed out
Does it act like a magnifying glass?
Does it focus light from the night light?
Refraction and Thin Lenses
Let’s try a simple (easy to make) shape:
SPHERICAL
Play with the lens that is handed out
Does it act like a magnifying glass?
Does it focus light from the night light?
Does the image distance depend on the shape
of the lens? (trade with your neighbor to get a
different shaped lens)
Property #5: 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
Double Slit Experiment
We will consider this situation
but only after we consider another:
the DOUBLE SLIT experiment:
Double Slit Experiment
Note that along the solid lines
are places where crests meets crests
and troughs meet troughs.
crest on crest
followed by
trough on trough
Double Slit Experiment
Note that along the dotted lines
are places where crests meets troughs
and troughs meet crests.
crest on trough
followed by trough on crest
crest on crest
followed by
trough on trough
Double Slit Experiment
Our question now is: How is the pattern
of bright and dark areas related to the
parameters of the situation: , d, x and L?
bright
x
dim

d
bright
L
dim
bright
SCREEN
Double slit: an example
n = d sin() = d (x/L)
d = 0.15 mm = 1.5 x 10-4 m
x = ??? measured in class
L = ??? measured in class
n = 1 (if x measured between adjacent bright spots)
 = d x / L = (you do the calculation)
Photoelectric Effect
Light hits a metal plate, and electrons are
ejected. These electrons are collected in the
circuit and form a light
current.
ejected electron
A
- +
V
Photoelectric Effect
The following graphs illustrate what the wave
theory predicts will happen:
I
current
I
current
I light intensity
Voltage
I
current
frequency of light
Photoelectric Effect
We now show in blue what actually happens:
I
current
I
current
V-stop
I light intensity
Voltage
I
current
f-co
frequency of light
Photoelectric Effect
In addition, we see a connect between V-stop
and f above fcutoff:
V-stop
fcutoff
frequency
Photoelectric Effect
• Einstein received the Nobel Prize for his
explanation of this. (He did NOT receive
the prize for his theory of relativity.)
Photoelectric Effect
• Einstein suggested that light consisted of discrete
units of energy, E = hf. Electrons could either
get hit with and absorb a whole photon, or they
could not. There was no in-between (getting part
of a photon).
• If the energy of the unit of light (photon) was not
large enough to let the electron escape from the
metal, no electrons would be ejected. (Hence, the
existence of f-cutoff.)
Wave-Particle Duality
The photo-electric effect can not be
understood by the wave theory, but can be
understood by the particle theory. Other
phenomena also are not described accurately by
the wave theory but are by the particle theory:
blackbody radiation, Compton scattering, the
sprectrum of hydrogen.
So, is light a wave or is it a particle?
More precisely, does light act like a wave or
does it act like a particle?
Wave-Particle Duality
Here is a rough analogy. (Remember the
strengths but also the weaknesses of analogies.)
Are you your mother’s son or daughter?
Are you a member of another group (sports
team, fraternity, sorority, etc?)
Do you act exactly the same way when with
your mother and with your group?
Are your actions fairly predictable when you
are with your mother and when you are with
your group?
Wave-Particle Duality
We notice that light behaves as a wave when
it is moving (refraction, double slit).
We also notice that light behaves as a particle
when it is created or when it “hits”
something (photoelectric effect).
Light is very predictable when viewed from
the Wave-Particle Duality theory.
Wave-Particle Duality
Can this strange wave-particle duality theory
“predict” new things to look for?
This wave-particle duality theory has been
developed to become the Quantum
Theory. It has predicted the Heisenberg
Uncertainty Principle, it has led to an
understanding of the Pauli Exclusion
Principle that explains the basis of
chemistry: why carbon is so different than
nitrogen or oxygen.