Light, Spectra, and Ma1er

Transcription

1 Light, Spectra, and Ma1er 1

2 Ideal Black Bodies Amount of light thermally radiated: j = σt 4 Where j = total intensity, T is temperature, and σ = 5.67 x 10-8 W m - 2 K - 4 (constant) Wavelength (nm) 2

3 SoluOon If star A is 9000 K and star B is 3000 K, what is the raoo of the power output of the stars (Power A /Power B ) assuming the stars are the same size? Intensity Intensity A B = P P A B / / A A = P P A B Intensity A = σt 4 A 3

4 SoluOon ( ) K 9000 K P P Intensity Intensity B A B A = = = = = = B A B A T T T T σ σ If star A is 9000 K and star B is 3000 K, what is the raoo of the intensity output of the stars (Power A /Power B)? 4

5 The Solar Spectrum: When we look at the Spectrum of the Sun, we see a distinct distribution of colors. Other stars have similar patterns as do most hot objects. The main difference is where the peak color is. Gustav Kirchhoff (1862) called this kind of emitter a Blackbody. 5

6 CalculaOng The Sun s Temperature So how well does this work? Pre1y well!!!! T Sun 6000 K 6

7 Fraunhofer s Surprise: Joseph von Fraunhofer made an interesong discovery when looking at the Sun. Using a precision dispersing prism, he discovered that the `solar black body` was cut by thousands of dark bands. 7

8 Fraunhofer s Surprise: Fraunhofer tried to test whether this effect was real. 1) He tested with different opocs. 2) He tested by looking at different objects (moon and planets). 8

9 Bunsen`s Solution: Robert Bunsen turned pyromania into one of the great discoveries of modern physics. Bunsen set fire to things in order to figure out what they were made of Gustav Kirchhoff suggested using a prism to break the light apart They quickly discovered that burning substances produced light in narrow bands with unique pa1erns Iron 9

10 Blueprint to Composition: Bunsen and Kirchoff`s trick was the key to finding out the composioon of anything from the light it produced. Many of the lines they found had the same wavelength as those of Fraunhofer`s dark bands. They were seeing the composioon of the Sun! 10

11 Kinds of Spectra: Bunsen found that he could idenofy the signature of different elements in the Fraunhofer spectrum of the Sun. Why were Bunsen`s spectra composed of bright lines while Fraunhofer`s were dark bands? Bunsen`s fires were somulaong emissions from thermal energy in the hot gas. So what are Fraunhofer`s bands? AbsorpOon in (and re- emission from) a cooler gas! 11

12 Types of Spectra Con)nuous: black body radiaoon con)nuous Absorp)on: requires a cool object in front of a hot background (ex: Fraunhofer) discrete Emission: requires a hot object with a cool background (ex: Bunsen) discrete 12

13 Types of Spectra Con)nuous: black body radiaoon con)nuous Absorp)on: requires a cool object in front of a hot background (ex: Fraunhofer) discrete Emission: requires a hot object with a cool background (ex: Bunsen) discrete 13

14 Atoms and Light: Why do elements have the `discrete` interacoons that Bunsen saw? Why do different elements (and molecules) have different interacoons? This has to do with the nature of atoms and how they are put together. 14

15 Dividing the Indivisible : The Plum Pudding Model 1897 J. J. Thomson discovers the electron (he called them corpuscles) Later, electrons are said to have charge = Thomson proposes model of the atom: Atom is divisible NegaOvely charged corpuscles/electrons (plums) PosiOve soup to balance negaove charge (pudding) 15

16 Basic DefiniOons: Element: a substance that cannot be broken down by chemical means (defined by number of protons) Atom: the smallest piece of ma1er that is soll an element Molecule: two or more atoms that are bound together by chemical bonds Nucleus: the protons and neutrons bound together at the center of an atom 16

17 Discovering Nothing Two new types of radiaoon discovered: 1. Alpha parocles (α): Helium nucleus 2. Beta parocles (β): electron Rutherford fires alpha parocles at gold foil. Expected only small angle sca1ering due to plum pudding. PosiOve Nucleus Saw mostly no sca1ering with occasional back sca1ering Ma/er is mostly empty space!!!! NegaOve electron 17

18 Rutherford s Atom 1. Mass (posiove nucleus) is highly concentrated at the center 2. Electrons (negaovely charged) circle the nucleus 3. Lots of empty space in between 4. Similar to today s atom Number of protons determine the element type Number of electrons determine the chemical properoes of the atom PosiOve Nucleus NegaOve electron 18

19 Inside the Nucleus Rutherford (1918): Discovers the proton. The proton is about 2000x as massive as the electron and has a charge = +1 James Chadwick (1932): Discovers the neutron. Neither posiove nor negaove, it has about the same mass as a proton Nuclei are made up of protons and neutrons. 19

20 Atoms, Elements, and Isotopes Atoms: Nucleus Protons number determines the element Neutrons number determines the isotope Electrons number determines the chemical properoes 20

21 Atoms, elements, and Isotopes Isotopes: Atoms with the same number of protons but different numbers of neutrons are called isotopes. Isotopes have the same chemical properoes, but different masses, different emission spectra, and parocipate in different nuclear reacoons. Hydrogen Deuterium TriOum Helium e - p + p + p + n p + e - First isotope of hydrogen n e - n n e - Second isotope of hydrogen p + n e - New element: not an isotope of H 21

22 Bohr Atomic Model Energy States: Wave properoes of electrons are important An integer number of wavelengths fit in one orbit p + Electron distances and energies are discrete values E1 E2 e - E ph = hf = hc λ E3 22

23 Bohr Atomic Model Wave properoes of electrons are important An integer number of wavelengths fit in one orbit Electron distances and energies are discrete values 23

24 Atoms and Light: Energy States: Electrons exist in `orbits` (much like planets in the solar system) that are stable at specific separaoons from the nucleus. (due to the wave- like nature of electrons) p + The distance from the nucleus determines the energy of the electron (lower E is closer). E1 E2 e - The spacing of these energy levels is not even. E3 E1è E2 > E2è E3 > E3è E4 etc 24

25 Atoms and Light: So what does all of this have to do with Bunsen and Franhofer lines? 25

26 Atoms and Light: Energy States: If you heat the atom up to high enough temperatures, the electron will jump to higher orbits (higher energy state). p + E1 e - How does `heaong` do this? Collisions E2 E3 26

27 Atoms and Light: Energy States: If you heat the atom up to high enough temperatures, the electron will jump to higher orbits (higher energy state). p + E1 How does `heaong` do this? Collisions E2 e - E3 27

28 Atoms and Light: Emission Energy States: Aqer a Ome, the electron falls back to the lowest energy state. A photon is given off. p + E1 E2 e - The energy of the photon is exactly equal to the energy difference between the two energy states. E3 28

29 Atoms and Light: Absorption Energy States: Process of emission is fully reversible. Electron can absorb a photon and jump to a higher energy level. p + E1 E2 e - The energy of the photon must be exactly equal to the energy difference between the two energy states. E3 29

30 Atoms and Light: Absorption Energy States: Process of emission is fully reversible. Electron can absorb a photon and jump to a higher energy level. p + E1 E2 e - The energy of the photon must be exactly equal to the energy difference between the two energy states. E3 30