What is Physics?

What is Physics?

First of all, Physics is a Science. So our first question should be: What is a Science?

Science

• 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 QuestionIs 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?

PhysicsNow 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


decide?

• If a wave, what is waving?

(waving even in a vacuum?)

Light


decide?

• If a wave, what is waving?

(waving even in a vacuum?)

Electric & Magnetic Fields

Properties of Light

• speed of light

• colors

• reflection

• refraction (bending)

• shadows

• energy theory

• absorption of light

• emission of light

Property 1: Speed of Light

• particle (photon) prediction?


• particle (photon) ? no prediction• wave (E&M) prediction?


• 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)


• 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/oo]

where the o and o are the electric and magnetic properties of vacuum.



in vacuum:

v = [1 / {o o}]1/2 where

o = 1/{4k} = 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



in material,

vphase = [1/oo]

= Ko , where K>1; and o ; so

v < c

According to the wave theory, light should move slower in material than in vacuum.


• 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: Colorexperiment ?

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) ? amount of energy • wave (E&M) ?

Property 2: Color

• particle (photon) ? amount of energy• wave (E&M) ? frequencyamong 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


• 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) ?


• experiment: objects in water seem closer than they really are when viewed from air

air

water

real object

apparentlocation

eye



water

air

surface

incident ray

refracted ray



water

air

surface

incident ray

refracted ray

vxi

vyi

vxr

vyr

vxi = vxr

vyi < vyr

therefore

vi < vr


• wave (E&M) ?

surface

air

water

incident wave

refracted wave

normal line

normal line

surface


• wave (E&M) ?

surface

air

water

incident wave

refracted wave

crest of wave

crest of preceding wave

x

a

w

normal line

crest of following wave

a

w


• wave (E&M) ? + = 90o

+ = 90o

surface

air

water

incident wave

refracted wave

crest of wave

crest of preceding wave

x

a

w

normal line

sin() = a /x

sin() = w /x


• 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!


• 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


• particle (photon) theory: vw > va

• wave (E&M) theory: vw < va

• experiment ?


• particle (photon) theory: vw > va

• wave (E&M) theory: vw < va

• experiment: vw < va

particle theory fails!

wave theory works!


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


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.


What kind of shape do we need to focus light from a point source to a point?

lens with some shape for front & back

screen

pointsourceof light

s = object distances’ = image distance


Let’s try a simple (easy to make) shape: SPHERICAL.

Play with the lens that is handed outDoes it act like a magnifying glass?


Let’s try a simple (easy to make) shape: SPHERICAL.


Does it focus light from the night light?


Let’s try a simple (easy to make) shape: SPHERICAL


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

lightdark dark

Light and Shadows

Consider what we would expect from

wave theory: shadows NOT sharp

lightdark darkdimdim

crest

crest

crest

Double Slit Experiment

We will consider this situation

but only after we consider another:

the DOUBLE SLIT experiment:


Note that along the solid lines

are places where crests meets crests

and troughs meet troughs.

crest on crestfollowed bytrough on trough


Note that along the dotted lines

are places where crests meets troughs

and troughs meet crests.

crest on crestfollowed bytrough on trough

crest on troughfollowed by trough on crest


Our question now is: How is the pattern

of bright and dark areas related to the

parameters of the situation: , d, x and L?

d

SCREEN

L

xbright

bright

dim

dim

bright

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 current.

A

light

+ -

V

ejected electron


The following graphs illustrate what the wave theory predicts will happen:

Icurrent

I light intensity

Icurrent

Voltage

Icurrent

frequency of light


We now show in blue what actually happens:

Icurrent

I light intensity

Icurrent

Voltage

Icurrent

frequency of light

V-stop

f-co


In addition, we see a connect between V-stop and f above fcutoff:

V-stop

frequencyfcutoff


• Einstein received the Nobel Prize for his explanation of this. (He did NOT receive the prize for his theory of relativity.)


• 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?


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?


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.


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.

Documents

What is Physics?