Chapter 27 Quantum Physics We now know that the world is different at high speed At very small sizes the world is also VERY different Started with light Is light a wave Yes because of the interference and diffraction Maybe not this simple Energy is discrete not continuous Everything is a probability nothing is for certain Particles often seem to be at two places at the same time Looking at something changes how it behaves i e nothing can be measured EXACTLY 27 1 Blackbody Radiation We know that hot objects emit light As the temperature increases Red hot White hot Blue hot These curves is explained ONLY if the energy of light is quantized E nhf h 6 626 X Visible Light max Joule sec Similar to electron charges e smallest charge Waves don t do this Blue Lowest wavelength is highest energy n number of PHOTONS Higher temperature more light emitted and peak intensity is lower higher energy maxT 0 2898 x 10 2 m K Can t be a fraction of hf A series of light bulbs are colored red yellow and blue Which bulb emits photons with the most energy Max Planck 10 34 Quantized there is a minimum amount of energy for light Wien s displacement law The least energy E hf hc Red Highest wavelength is lowest energy UV energy higher than blue it hurts 1 Example A red and green laser are each rated at 5mW Which one produces more photons second photons Energy second Power Power second Energy photon Energy photon hf Lesson learned in Relativity Modern physics is based on observations with modern techniques not our own experience All bets are off Don t think why it behaves this way God made it this way Accept what happens and find its rules Red light has less energy photon so if they both have the same total energy red has to have more photons 27 2 Photons and the Photoelectric Effect Light shining on a metal can knock electrons out of the metal Photoelectric Effect Each metal has Work Function W 0 which is the minimum energy needed to free an electron from the metal Light comes in packets called Photons E hf Not determined by light intensity but by photon energy i e not by brightness but by color i e energy Collision of ONE photon with ONE electron h 6 626 X 10 34 Joules sec Maximum kinetic energy of released electrons KE hf W 0 This equation rather than the beautiful theories of relativity won Einstein his Nobel prize 2 Example When a 500 nm light hits a metal the stopping potential is 1 0 V What is the stopping voltage when a 400 nm light hits the metal The experimental setup KE hf W 0 Electrons are ejected from the metal plate E Electron flow to E would give a current Use a voltage to stop the electrons When Vs e KEmax no electron can reach C and current 0 W0 hf Vs e KEmax Stopping potential 27 6 The Due Nature of Light Matter A red photon has energy than a blue photon 1 More 2 Less 3 Same If you were a French physics grad student you can get away with a few things de Broglie a French physics grad student sneaked in a few things in his thesis All matter including you and me are both matter and wave at the same time h For a photon the wavelength is p and therefore it is the same for particles Reason He felt it would be nice if that was true more consistent with light Experimental evidence none 3 People were getting worried with the wave matter business nothing seemed to be obvious Are electrons really particles Particles definitely particles You can bounce things off them How would we know if electron is not exactly a particle Learn from the photons look for interference Examples If an electron moves at speed of 1 m s what is its wavelength h p p mv 9 1 x 10 31 x 1 9 1 x 10 31 kg m s h p 6 626 x 10 34 9 1 x 10 31 7 3 x 10 4 m With today s tools it is very easy to detect If someone m 66 26 kg is walking at 1 m s what is the person s wavelength h 6 626 x 10 34 J s Wave picture was observed in experiments many years after de Broglie s assumption 27 7 The Wave Function A probability function Electron cloud Orbital the probability distribution of one electron What you see in chemistry books is only the most probable part of the orbital All orbitals extend to infinity with decreasing probability An electron is not at one place for a given moment in time but at all possible places in the orbital 4 Scanning Electron Microscopy Resolution 5 nm resolution of optical microscopes 100 nm Up to 10 000 V greater momentum shorter wavelength 27 6 The Heisenberg Uncertainty Principle The smaller the probe compared to the object the better the accuracy What if the object is so small that even light can disturb the object Transmission Electron Microscope 109 Fronczak Hall V 200 000 V Electron velocity 0 65c relativistic 2 8 10 12 m Resolution 0 1 nm Uncertainty principle h p x x 4 h E t 4 Not only we cannot measure anything exactly trying hard to measure one accurately also messes up another one When the precision of one gets better the other gets worse We can never measure anything exactly We know exactly what we don t know almost 5 If h 100 your seat is 0 5 m wide what is your uncertainty of velocity assuming m 50 km 27 9 The Scanning Tunneling Microscope 140 Fronczak Hall Resolution 0 001 nm the end of the stable world How about even smaller things like an electron 10 6 nm or a nucleus 10 5 nm the definition of radius becomes fuzzy High energy accelerators a couple of km in diameter not direct imaging but enough info for physicists to construct the internal structures of particles like electrons 6