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PHYS 1200 FINAL EXAM FINAL EXAM Wednesday December 17 12 30 P 2 30 P in LR 1 VAN FE covers Lectures 23 36 The study guide formulas and practice.final exam questions are posted on theExam Information Link below We will review the practice final examquestions on Wed Dec 10 and Friday L 33 Modern Physics 1 . Introduction quantum physics Particles of light PHOTONS The photoelectric effect Photocells intrusion detection devices The Bohr atom. emission absorption of radiation LASERSSometimes light behaves like a particle andsometimes particles behave like waves Modern Physics Introduction. Modern 20th Century By the end of the 19th century it seemed thatall the laws of physics were known planetary motion was understood the laws of electricity and magnetism were known. the conservation principles were established However there were a few problems whereclassical physics didn t seem to work It became obvious that Newton s laws couldnot explain phenomena at the level of atoms 3. ATOMS and classical physics In the classical picture the electrons in atoms orbit aroundthe nucleus just as the planets orbit around the Sun However the laws of mechanics and electromagnetismpredict that an orbiting electron should continually radiate.electromagnetic waves and very quickly the electronwould loose all of its energy and collapse into the nucleus Classically there could be no atoms Problems with Newton s Laws Newton s laws which were so successful in.allowing us to understand the behavior of bigobjects such as the motions of the planets couldnot explain phenomena at the atomic level This is not too surprising since Newton s lawswere discovered by considering the behavior of.macroscopic objects like planets Physical laws have a limited range ofapplicability and must be continually testedto find their limitations and then modified Newton s laws fail at high.velocitiesaccelerate to K measure vpredictionElectron velocity cRelativistic.prediction Einstein showedthat mass is not aconstant butdepends on speed DATA. As speedincreases so does mass Speed can never Kinetic Energy J exceed the speed 6. The failure of the old physics We will now discuss an example of an effect thatcould not be explained by the pre 20th centurylaws of physics The discovery of the correct explanation led to a.revolution in the way we think about light andmatter particles and waves The new concepts also led to a revolution intechnology that has changed our lives e g thesemiconductor led to the introduction of the.personal computes cell phones etc The photoelectric effect photonsphotoelectronsMetal plate When light shines on a metal surface electrons may. Photoelectrons are only emitted if the wavelength ofthe light is shorter than some maximum value nomatter how intense the light is so the color wavelength is critical blue light makes electrons pop out red light does not. Details of a photocell Photocells used as a safety deviceThe child interrupts the beam stoppingthe current which causes the motor to No classical explanation for the.photoelectric effect According to electromagnetic wavetheory if the intensity of the light issufficiently high the electron should beable to absorb enough energy to escape. The wavelength of the light should notmake a difference But the wavelength does matter Einstein received the 1921 Nobel Prizefor explaining the photoelectric effect. A radical idea was needed to explain thephotoelectric effect Light is an electromagnetic wave but when itinteracts with matter the metal surface itbehaves like a particle. Light is a particle called a photon packetsof energy moving at the speed of light A beam of light is thought of as a beam of Photoelectric effect PHOTONS The energy of a photon depends on the.wavelength or frequency of the light Recall that speed of light wavelength l x frequency f Photon energy E h fE Planck s constant h x frequency h f.h 6 626 x 10 34 J s f c l E h c l hc l Shorter wavelength or higher f photons have ahigher energy The photon concept explains the.photoelectric effect A certain amount of energy is required toremove an electron from a metal A photoelectron is emitted if it absorbs aphoton from the light beam that has.enough energy high enough frequency No matter how many photons hit theelectron if they don t have the right energythe electron doesn t come out of the metal Blue and red photons example. How much energy does a photon of wavelength 350 nm nanometers have compared to aphoton of wavelength 700 nm Solution The shorter wavelength photon hasthe higher frequency The 350 nm photon has.twice the frequency as the 700 nm photon Therefore the 350 nm photon has twice theenergy as the 700 nm photon The quantum concept The photon concept is a radical departure.from classical thinking In classical physics energy can come inany amounts In modern physics energy is QUANTIZED comes in definite packets photons of.energy h f In the PE effect energy is absorbed by theelectrons only in discreet amounts Video recorders and digital cameras Electronic cameras convert light into an electric charge.using the photoelectric effect A two dimensional megapixel array of sensors capturesthe charge and records its intensity on computer memory Niels Bohr explains atoms in 1913 Niels Bohr a Danish physicist used.the quantum concept to explain thenature of the atom Recall that the electron in a hydrogenatom should quickly radiate away allof its energy. If this occurred atoms would emitradiation over a continuous rangeof wavelengths But atoms emit light in discreet lines Line spectra of atoms.Line spectra are like fingerprints whichuniquely identify the atom 19 The Bohr Atom The electrons move incertain allowed stationary .orbits or states in whichNucleus then do not radiate The electron in a highEf energy state can make atransition to a lower energy.state by emitting a photonwhose energy was theThe orbits farther from difference in energies of thethe nucleus are higherenergy states than two states hf Ei Ef.the closer ones 20 Line spectra of atomic hydrogenThe Bohr model was successful in predictingwhere all the spectral lines of H should be Emission and Absorption. When an electron jumps from a highenergy state to a low energy state itemits a photon emission spectrum An electron in a low energy state canabsorb a photon and move up to a high.energy state absorption spectrum Emission and AbsorptionElectron spontaneously Electron absorbs ajumps to a lower energy photon and jumps tostate and emits a photon a higher energy state.PHYS:1200 FINAL EXAM. FINAL EXAM: Wednesday December 17, 12:30 P - 2:30 P in LR-1 VAN . FE covers Lectures 23 â€“ 36. The study guide, formulas, and practice final exam questions are posted on the Exam Information Link below. We will review the practice final exam questions on Wed. Dec. 10, and Friday Dec. 12.