Photoelectric effect demo

0V ?V

If I add a bunch of extra electrons to a capacitor plate is the

voltage on that plate positive or negative?

A) + (positive)

B) - (negative)

C) Depends on what the word is means

Photoelectric effect demo

0V ?V

If I add insert an electron into the gap which of the following

will happen?

A) Electron will be attracted to the charged plate

B) Electron will be repelled from the charged plate

C) Electron will dissappear

Free electron introduced near

charged electrode

Charged metal will repel itself

-V

In an electroscope a metal is charged. The metal has a thin

piece attached which is light and flexible.

When it is charged the flexible piece is repelled.

If we then use the photoelectric effect to eject electrons they

will be repelled and the charge will go away.

-V

Today

Photons and Atoms

HW5 Due Wednesday. I will not have office hours tomorrow (Tuesday) Instead I will have extra ones:

Today 4-5 office (F619)

And tomorrow morning 10-11am help room.

Reading 5.1-5.3

Review: PE sim. That's what we found:

0 U

I

1. Current vs. Voltage:

0 Frequency

I

2. I vs. f:

0 Frequency

Init

ial K

E

or: Initial KE vs. f:

0 Intensity

I

3. I vs. intensity:

low intensity

high intensity Threshold

Threshold Threshold

Classical wave predictions vs. experimental observations

• Increase intensity current increases.

Experiment matches with classical prediction

• Takes time to heat up ⇒ if thermal effect, current would

initially be low and increase with time.

Experiment: electrons come out immediately, no time

delay to heat up

• Classical: Color of light does not matter, only intensity.

Experiment shows strong dependence on color

• Current vs. voltage: step close to zero Volts, then flat.

Flat part matches to classical pred., but experiment

has 'tail' of energetic electrons Stopping potential,

which depends on color (not only intensity). Also

stopping potential linear in frequency!

What could it be?

The PE effect is inconsistent

with classical E&M theory!!

PE effect: Discovered 1887 by Hertz, 1905 Explained by

Einstein, using some of Plank's ideas. Nobel prize: 1921

“…”

Einstein proposed:

"…the energy in a beam of light is not distributed

continuously through space, but consists of a finite

number of energy quanta, which are localized at points,

which cannot be subdivided, and which are absorbed

and emitted only as whole units." He took the energy of

these single units to be hf, as proposed earlier by Planck.

Doesn’t look like a wave to me… Seems more like a particle!!

Is light a stream of particles?

Yes! Also….

E = hf

Ekin,max=hf -

“Work function” (I’ll explain later)

The energy of a photon is E = hf

The wavelength of a photon is λ = c/f = hc/E

The momentum of a photon is p = E/c = hf/c

The mass of a photon is m = 0

h ≈ 6.626 ·10-34 J·s: Plank constant

It sometimes is useful to define h = h/(2π)

The energy of a photon is then: E = hf = hω

Properties of photons

The frequency of a beam of light is decreased but the light’s intensity remains unchanged. Which of the following is true? A. There are more photons per second but each

photon has less energy. B. There are more photons per second and each

photon has more energy. C. There are fewer photons per second and each

photon has less energy. D. There are fewer photons per second but each

photon has more energy. E. Nothing happens to the photon number because

light is a wave.

Photons

What actually happens in the metal

when a photon strikes?

Why do the emitted electrons have

different velocities/kinetic energies?

What determines the work function ‘Φ’?

What happens in the metal? Kicker analogy: Photon is like a kicker in a pit…

Puts in energy. All concentrated

on one ball/electron.

Blue kicker has a fixed strength.

KE = kick energy - mgh

Ball emerges with:

mgh = energy needed to

make it up hill and out.

mgh for highest electron

analogous to work function.

Fixed kick energy:

Top ones get out…

…bottom ones don’t.

h

metal

electrons

Red kicker (photon) kicks less

than blue one. Nothing gets out.

‘Photon’ ‘Electron’ For electrons:

KE = hf - Φ

Φ

Different metals different ‘pit depths’

sodium- easy to kick out

platinum, hard to kick out

large work function deep pit

Φ

small work function shallow pit

Φ

PE effect: Apply Conservation of Energy

Inside

metal

Ele

ctr

on P

ote

ntial

Ene

rgy

work function () = energy needed to kick highest electron out of metal

Energy of photon = energy needed to kick KE of electron

electron out of metal as it exits the metal

Loosely stuck electron, takes least energy to kick out

Tightly stuck, needs more

energy to escape

Outside the metal

Energy in = Energy out

Φ

Energy in = Energy out

+

“Fermi” level (weakest bound electron)

Inside

metal

Ele

ctr

on P

ote

ntial

Energ

y

work function ()

Outside

metal

What happens if send in bunch of blue photons?

Electrons have equal chance of absorbing photon:

Max KE of electrons = photon energy -

Min KE = 0

Some electrons, not enough energy to pop-out, energy into heat.

Ephoton

Photon gives electron “kick of energy”.

Energy of photon = energy needed to kick + Initial KE of electron

electron out of metal as exits metal

Energy in = Energy out

PE effect: Apply Conservation of Energy

Electrons over large range of energy have equal

chance of absorbing photons.

Inside

metal

Ele

ctr

on

pote

ntial

energ

y

You initially have blue light

shining on metal. If you

change the frequency to

violet light (at same # of

photons per second), what

happens to the number of

electrons coming out?

work function

Ephot

Ephot

a. fewer electrons

b. same # of electrons but electrons are faster

c. more electrons

d. not enough information

Electrons over large range of energy have equal

chance of absorbing photons.

metal

ele

ct.

po

ten

tia

l

energ

y

work function

Ephot

c. more electrons come out with violet

absorb blue light and have enough energy to leave

absorb blue light, but don’t come out

so the more energy the light has, the more electrons that come

out, until so much energy that every electron comes out.

(violet and ultraviolet would not be very different in this case)

photoelectric.jar

Typical energies

Each photon has: Energy = Planks constant * Frequency

(Energy in Joules) (Energy in eV)

E=hf=(6.626*10-34 J-s)*(f s-1) E=hf= (4.14*10-15 eV-s)*(f s-1)

E=hc/l = (1.99*10-25 J-m)/(l m) E= hc/l = (1240 eV-nm)/(l nm)

Photon Energies:

Work functions of metals (in eV): Aluminum 4.08 eV Sodium 2.28 Lead 4.14 Potassium 2.3

Beryllium 5.0 eV Cobalt 5.0 Magnesium 3.68 Platinum 6.35

Cadmium 4.07 eV Copper 4.7 Mercury 4.5 Selenium 5.11

Calcium 2.9 Gold 5.1 Nickel 5.01 Silver 4.73

Carbon 4.81 Iron 4.5 Niobium 4.3

Red Photon: 650 nm Ephoton = 1240 eV-nm = 1.91 eV

650 nm

Recap: Intensity Emax2 but does not depend on frequency!

Interference was definitive test that light is a wave!

“Why do higher frequency gamma rays carry more energy than

lower frequency radio waves, but frequency has nothing to do

with intensity, what gives?”

“But Al said that energy of light depends on frequency ( hf)!”

Do not confuse the energy carried by a beam of light

with the energy of a single quantum particle of light!

X

X

vs.

Common confusion:

Electromagnetic Waves and Photons

are describing the same thing! Need to have a model where these views/perspectives all fit together

and are consistent with each other!

This picture can explain wave view and particle view.

“Particle” = little chunk of the electromagnetic wave.

Energy of photon (hf) is in its oscillating E and B fields. (Sometimes it also helps to think of a photon as a tiny particle/energy packet).

1. Light, Radio waves, X-rays are all electromagnetic waves!

2. The electromagnetic wave is made up from lots of photons.

3. Photons can be thought of as mini E/M wave segments

(each has a specific energy hf and a wavelength c/f )

Travels

Straight

E Electromagnetic

Wave

Made up from

photons

Made up from

lots of photons

If you think of photons as particles you probably

automatically think of them as perfectly localized - like a

classical billiard ball at a coordinate (x, y, z).

This is what get's you into trouble in QM!!

• Classical Billiard balls never produce a diffraction pattern

• Classical Billiard balls have no wavelength/frequency

• Classical Billiard balls cannot go through two slits at the

same time (photons can; electrons too! Will show later)

When is it important to think

about particle aspect of light?

Only if your “detection system” is good enough to see

individual photons!

Examples where don’t need to think about particle behavior

(detection system isn’t good enough and wave behavior is easier):

Heating: Energy absorbed in microwave or by black asphalt.

Optics: Light bending through lenses or passing through slits

Laser beam: Treat it just like a beam of light… (Understanding

the working principle of a laser requires photon picture.)

Examples where important to think about particle behavior:

Photoelectric effect: Individual electrons popping out of metal

Lasers: Electrons in atoms transitioning between energy levels

Molecular bonds: Chemical bonds breaking