Chapter 27- Atomic/Quantum Physics

The Sun http://soho.nascom.nasa.gov/ Why do we see the sun as yellow instead of green or blue or pink?

Blackbody Spectrum

Photon Theory of Light Light is transmitted as tiny particles called photons The amount of energy in a photon depends on its frequency h= Planck’s Constant= 6.626 x 10-34 Js f= frequency of light c= speed of light λ= wavelength p= momentum

The Photoelectric Effect When light shines on a metal surface, electrons are emitted from the surface

Photocells (p.829) When the photocell is in the dark, the ammeter reads 0 (no current) When light with a high enough frequency shines on the current flows in the circuit

Photocells KEmax of the emitted electrons can be found by reversing the voltage and making the C electrode negative The electrons are repelled by C, but the fastest electrons will still make it across There is a minimum voltage, Vo, called the stopping voltage. No current will flow if the voltage is less than the stopping voltage Kemax = e Vo

Photoelectric Effect Einstein’s Theory Predicts: Increasing the intensity of the light does not make the electrons go faster. This is because although more photons are striking the surface, they have same energy Increasing the frequency of the light beam increases the energy of the photons which changes the maximum KE of the ejected electrons

Photoelectric Effect The work function, Wo, is the minimum amount of energy necessary to get an electron off the surface of the metal fo is the “cutoff” frequency. If the light beam’s frequency is below that, then no electrons will be emitted

Compton Effect A.H. Compton scattered xrays from various materials Found out that the scattered light had a lower frequency than incident light Since frequency decreases, wavelength increases Used conservation of momentum to determine that the photon transfers some of its energy to the electron

de Broglie Wavelength Light sometimes behaves like a wave and sometimes like a particle Louis de Broglie came up with the idea that particles might also have wave properties De Broglie wavelength of a particle

de Broglie Wavelength The wavelength of large objects is very small For a 0.20 kg ball travelling at 15 m/s

de Broglie Wavelength Determine the wavelength of an electron that has been accelerated through a potential difference of 100 V

Davisson-Germer Experiment The spacing of atoms in a crystals is on the order of 10-10 m, so one could be used as a diffraction grating In 1927, Davisson and Germer scattered electrons from the surface of a metal crystal. The wavelength they got matched the predicted de Broglie wavelength

Bohr Model of the Atom Electrons orbit the nucleus in circular orbits called stationary states When an electron jumps from one state to another, light is either absorbed or emitted The energy required to go between states is a fixed amount

Bohr Model of the atom If an electron jumps from a higher state to a lower state, it emits a single photon of light

Energy Level Diagram (p. 847) n= 1is ground state, n=2,3,4.. Are excited states To completely free an electron in the ground state, you’d need to put in 13.6 eV of energy (ionization energy of Hydrogen)

Energy Level Diagram How much energy to go from ground to n=2? How much energy to go from n=2 to n=4?

Energy Level Diagrams What are the possible transitions for an electron in excited state n=3? 31 32 21

Emission Spectra A material’s emission spectrum show the wavelengths of the photons emitted when electrons jump to lower energy states

Absorption Spectra The absorption spectrum of a material shows that gases can absorb light at the same frequencies at which they emit

Absorption/Emission Spectra