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 WavesParticle ReflectionYes RefractionYes InterferenceYesNo DiffractionYesNo PolarizationYesNo Photoelectric EffectYes

The Photoelectric Effect If we shine light onto the surface on a metal electron are ejected. base.htmlhttp://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. – eriment%201http:// eriment%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 = 1.60x J thus 2.11 x 1.60x = 3.38x 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… 3.38x J = 6.63x Js x f f = 5.10 x 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 f o, if graphed, the slope will represent planks constant (h), the x intercept is f o –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/λ The 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)

E photon = E initial -E final 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 = (-3.40) = eV What 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 = eV – (-13.6 eV) = eV Energy in joules = eV  1.6  J/eV = 1.93  J Use  = hc = 6.63  Js  3.0  10 8 m/s E 1.93 x J  1.03  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 ? Mev Mev Mev 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 the n = 2 level is excited further until the electron is in the n = 4 level. This energy level change occurs 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 = KE max + W 0 –Lambda = h / c * lambda (de Broglie) –Stopping Potential e V 0 = 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 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 = mc 2 Universal mass unit Fission vs. Fusion