04/19/23 Lecture X 1

Physics 123

Photoelectric effect, photons

04/19/23 Lecture X 2

Concepts

• Photoelectric effect

• Work function and stopping potential

• ElectronVolt (eV)

• Photons

• Blackbody radiation

04/19/23 Lecture X 3

Photoelectric effect• Light shines on metal surface P• Electrons are emitted from P• Current flows between P and C• Measure kinetic energy of emitted

electrons KEe by applying a stopping potential V0:

KEe=eV0

• No surprises so far, because light as a wave carries energy

04/19/23 Lecture X 4

Electronvolt

• Energy that one electron gains when being accelerated over 1V potential difference is called electronvolt eV:

• 1eV=1.6x10-19C 1V= 1.6x10-19J• Yet another unit to measure

energy,• Commonly used in atomic

and particle physics.eVEnergy 5000

04/19/23 Lecture X 5

Photoelectric effect• Inside metal electrons are

sitting in potential “wells”• Need supply some minimum

energy W0 to get them out

• W0 is called work function – different for different materials

• Light provides this energy EL

EL=Ne(W0+KEe)

• Ne- number of emitted electrons

e-W0e-

light

04/19/23 Lecture X 6

Photoelectric effectEL=Ne(W0+KEe)

• Here is a surprise:

• Increase light intensity EL– expect

– More electrons Ne - True

– With higher kinetic energy KEe – False

• Do not expect KE to depend on light wavelength , but it does– More over if >0 – no electrons come

out no matter how intensive the light source is!!!

04/19/23 Lecture X 7

Photoelectric effect• Explanation of photoeffect

A.Einstein

• Postulate: light is transmitted in tiny particles (!!) – photons ()

• Each photon carries energy proportional to its frequency:

E=hf=hc/• Planck’s constant h=6.626 x 10-34 J.s

• hc=1243 eV.nm

• One electron absorbs only one photon

• If E<W0 – no electrons are emitted:

• E<W0 f<f0

e-W0e-

E E E

f0=W0/hhc/W0

04/19/23 Lecture X 8

Photoelectric effectE=hf=hc/

• Photon energy (E) is spent to get electron out (W0) and on electron’s kinetic energy (KEe):

E= W0+ KEe

• If stopping potential is applied electron’s kinetic energy KEe is converted into potential energy of the electron eV0:

E= W0+ eV0

e-W0e-

E E E

KEe

04/19/23 Lecture X 9

Photons – particles of light• Photon= particle of light = quantum of light =

gamma()-quant

• Intensity of electromagnetic wave – sum of energies carried by quanta of light – photons:

I=NE

• Each photon carries energy proportional to the frequency of the EM wave:

E=hf=hc/

04/19/23 Lecture X 10

Problem 38-15

• I=0 when >nm• Work function W0-?

• What stopping potential (V0) must be applied if light of =400 nm is used?

04/19/23 Lecture X 11

Photon absorption and emission• Photons can be absorbed by matter – converted into other

forms of energy – e.g. kinetic, potential, thermal. At this point photon ceases to exist

Final energy=Initial energy+E

• Photons can be emitted thus reducing the energy of the remaining system. Photon was NOT hiding inside matter waiting to be released, it is created by converting other forms of energy into light (EM-wave)

Final energy+E=Initial energy

• Energy is conserved in either case

04/19/23 Lecture X 12

More particle properties of light• Sun tan –

– chemical reaction with threshold energy W0

– visible light =700-400 nm does not have enough energy to start this reaction

– ultraviolet – <400 nm - more energetic light – does have enough energy

• Photographic film exposure– Why red light is safe?

e-W0e-

E E E

KEe

04/19/23 Lecture X 13

Blackbody radiation• Another problem was elegantly solved by introducing

“light particles”• Classical explanation of Blackbody (no reflection

radiation diverged at low wavelength• Max Planck suggested replacing integral over

frequencies with summing a series (photons!)

peak pT=2.9x10-3 mK• Planck did not seek deep meaning behind this seemingly

mathematical trick• Fundamental explanation was suggested by Einstein in

1905 – energy is quantized