 Atomic Structure  Protons and neutrons make up the nucleus  Electrons orbit the nucleus at specific distances known as energy levels – unique to each atom Atomic number – number of protons = Z Atomic mass number – A=Z+N, where n is the number of neutrons Atomic mass – total mass of the electrons, protons and neutrons Atomic weight – the ration of the average mass of an atom to 1/12 of the mass of an atom of carbon-12 Ion – if an atom loses or gains an electron and has a net charge Isotope – atom has the same number of protons but different number of neutrons Emitting Light Electrons can change their energy levels by absorbing or emitting energy, the form of light or what are called protons In order to change the energy level the energy of the absorbed or emitted photon must be exactly equal to the energy difference between the energy levels.  Now for the light, the energy is related to the wavelength and frequency: E=h = hc/ν λ So the absorbed and emitted light is at a specific wavelength To get white light, we need a large collection of different atoms emitting light at different wavelengths Message of light Light can tell us about composition of the object producing it How? - via a process called spectroscopy Use a prism or a grating and pass light through it to create a spectrum (distribution of the energy of the light as a function of wavelength) Since photons from each atom are all emitted at different wavelengths, they are unique. If you see light at a specific wavelength corresponding to a transition in a particular atom, the atom is present in the object emitting the light.  Different kinds of Spectra Emission – the spectrum contains only bright lines Absorption – the spectrum contains “dark lines” places where the continuousemission is much much much less than the surrounding areas. Continuum spectrum – a continuous distribution of energy at all wavelengths.  Kirchoff’s Laws Any opaque solid, liquid or gas produces thermal radiation with a continuousspectrum. A thin gas will produce an emission (bright line) spectrum. A thin, cooler gas in front of a continuous source will produce an absorption (dark line) spectrum. Blackbody Radiation A blackbody is an idea emitter of light It emits some light at all wavelengths of the electromagnetic spectrum Proposeed by Kirchoff to explain why heated bodies radiated Modeled it as an oven with a small hole-led to this sometimes being called hole radiation Observations of hole radiation P= Tσ4 The total power radiated from one square meter is proportional to T4 . isσthe stefan – boltzmann constant and = 5.67 x 10-8 w/m2 K4 Known as Stefan’s Law λmax= 2.0 x 106 /T The wavelength at which the hole (Blackbody) emits its maximum amount of radiation is inversely proportional to its temperature. Known as the Wien radiation Law How to explain the black body curve Using a classical wave theory of light approach, Rayleigh tried to formulate a mathematical description of the radiation in the oven. His formula, known as the Rayleigh-Jeans formula:  I λ(T) = 2ckT/λ4 This works great at long wavelengths, but diverges significantly at short wavelengths; by the ultraviolet it indicates infinite energy density. Became known as the ultraviolet catastrophe.  Planck’s Solution Planck set out to find a function that fit the observed blackbody curve. He found: I( ) = 2hcλ2/λ5{1/(e(hc/ kt)λ-1)} Known as the Planck Radiation law But Planck then wanted to explain why this particular function worked! He had to assume that the oven was not filled with a continuous distribution of waves, but with individual oscillators who could only radiate at discrete (quantized) energies given by E=hc/ =h .λ ν Stars emit like blackbodies When the flux of density of a star is plotted across the e-m spectrum, it resembles a black body So the Planck radiation law not only describes hot ovens with holes in them itdescribes stars That means the Wien Law λmax= 2.9 x 106 /T describes a star’s