Modern Physics

•Previously we showed that Light behaves like sound. It has characteristics of waves

•Now we get to see how it also behaves like a particle

•Think about what happens when we turn up the volume of our MP3 players in terms of energy

How do we define light

Phenomenon Waves Particle

Reflection Yes Yes

Refraction Yes Yes

Interference Yes No

Diffraction Yes No

Polarization Yes No

Photoelectric Effect Yes

The Photoelectric Effect

• If we shine light onto the surface on a metal electron are ejected.

• http://phet.colorado.edu/web-pages/simulations-base.html

Wave - Particle Duality• So what are we shooting at the metal to eject

electrons?• What particles make up light?

– Photons

– The basic unit of electromagnetic energy is known as a photon. A photon is a massless particle of light that carries both energy and momentum.

– http://www.physchem.co.za/Light/Particles.htm#Experiment%201

– What is an eV ? The amount of energy required to move an electron through 1 volt

How much energy is in a Photon?

Sample Problem:

• What is the energy, in Joules of a photon whose energy is 2.11 electronvolts?

– Answer: 1 eV = 1.60x10-19 J thus 2.11 x 1.60x10-19 =

• 3.38x10-19 J• What are the units?

– What variable does this represent

– What kind?

Light and Energy

• Using the Energy (E) calculated form the previous page, can we now calculate the frequency of the photon in the example? Yes!

E = hf so…

3.38x10-19 J = 6.63x10-34 Js x f

f = 5.10 x 10 14 Hz

What color is this?

Determining the work function

• If we plot Frequency vs K.E. we get a specific graph for each metal

Metal 2

Threshold Frequency

• Will all metals give off electrons with the same frequency of light?– No way, nada, nunca

• Each metal has a specific threshold frequency fo, if graphed, the slope will represent planks constant (h), the x intercept is fo

– Increasing the intensity will not effect the number of e emmitted

• Note: this contradicts the wave theory of light

Energy of a Photon

• The energy for each individual photon can be calculated using…

E photon= hf or = hc/λThe equation states that the energy of a photon is directly

proportional to the frequency and inversely proportional to the wavelength

• So what is h?

– Planks constant guess where it can be found

Momentum of a photon

This is very important, so listen! When a photon of visible light, strikes a metal surface, the photon’s energy is completely absorbed and transferred to the emitted electron. When an x-ray photon and an electron collide, some of the energy of the photon is transferred To the electron, and the photon recoils with less energy, less energy means means that the photon now has a lower frequency.What does this all mean?

The collision causes a conservation of energy

The photon loses energy and momentum while the electron gains energy and momentum. (compton effect)

Ephoton = Einitial -Efinal Using the formula above, it can be determined if energy was

absorbed or given off by an electron or photon. If the calculation above is a positive number, then the atom emitted a photon, if the calculation yields a negative number, the atom absorbed a proton.

Example:

What is the energy given off by a hydrogen atom if the electron jumps form the 6th energy level back to the 2nd energy level?Solution:

Ep = -.38 - (-3.40) = + 3.32eVWhat color light would this emit with this jump?

Mysterious light• When we view white light through a diffraction grating what do we

see?– A rainbow of course

– Specifically, this is the color spectrum (continous)

• Why then do we only see lines when viewing gasses?

•Niels Bohr studied this an concluded:

•an atom can only absorb certain energies (colors) of light (the absorption spectrum) and once excited can only release certain energies (the emission spectrum)

The Bohr Model• Bohr used these observations to argue that the

energy of a bound electron is limited certain to quantities of energy.

• This was given the term "quantized."  • Since only certain energy levels are allowed it is

actually possible to diagram the atom in terms of its energy levels

The energy states of the electron depend upon its particular orbit.

When an electron is in a particular level, it is in a stationary state.. Each stationary state represents a particular amount of energy and is known as the energy level.

Ground state

Excited state

The n1 state is the ground state, all of the energy levels above this are known as excited states. Atoms rapidly lose the energy of their various excited states and return to the ground state. The lose of photons of specific frequencies causes spectrum lines characteristic to each element.

Using energy level diagrams• What is the

wavelength of photons of light given out by the transition from –1.51 eV to the ground state (-13.6 eV)?

What wavelength of light is emitted?

• Energy given out = • -1.51 eV – (-13.6 eV) = 12.09 eV

• Energy in joules = • 12.09 eV 1.6 10-19 J/eV = 1.93 10-18 J

• Use = hc = 6.63 10-34 Js 3.0 108 m/s

E 1.93 x 10 -18 J1.03 10-7 m

• If a deuterium nucleus has a mass of 1.53 x 10-3 universal mass units less then its components, this mass represents an energy of ?

1. 1.38 Mev

2. 1.42 Mev

3. 1.53 Mev

4. 3.16 Mev

More• Determine the frequency of a photon

whose energy is 3.00 10–19 joule

• If a proton were to combine with an antiproton, they would annihilate each other and become energy. Calculate the amount of energy that would be released by this annihilation.

A hydrogen atom with an electron initially in then = 2 level is excited further until the electron

isin the n = 4 level. This energy level changeoccurs because the atom has(1) absorbed a 0.85-eV photon(2) emitted a 0.85-eV photon(3) absorbed a 2.55-eV photon(4) emitted a 2.55-eV photon

Chapter 27 Summary

• Major Equations: c = F * lambda– E = hf (h = planks constant)– P = h / lambda

– E = KEmax + W0

– Lambda = h / c * lambda (de Broglie)

– Stopping Potential e V0 = KE max

Major Concepts

• Photons – Can knock an e out of an atom– Can collide with an electron and lose energy– Can knock an electron to a higher energy level– Can vanish and produce matter/anti matter pair

Problems of interest

• Photoelectric problems

• De Broglie wavelength for matter

• Compton Scattering

• Atomic energy level problems

History of the atom

• Thompson (1897) plum pudding• Millikan (1913) e = 1.6 x 10-19 J• Einstein (1905) E = hf = Ke max Epdepends on Freq• Compton (1923) photons have momentum• deBroglie (1923) Particles act like matter• Rutherford (1911) nucleus of atom• Bohr (1913) planetary model energy levels• Heisenburg (1925) delta x delta p >= 2pie /h

Last Topic

• Nuclear Physics chapter 30 in text

• Please read and pay attention to decay series

• Major concept is E = mc2

• Universal mass unit

• Fission vs. Fusion