Astro110-01 Lecture 14 Light and Matter 20/02/09 - PowerPoint PPT Presentation

Astro110-01 Lecture 14 Light and Matter 20/02/09 Habbal_Astro110-01 Lecture 14 1

What are the building blocks of matter? Or the parts of an atom Electron cloud Nucleus Atom proton neutron mass (proton) ≅ mass (neutron) ≅ 2000 x mass(e) 20/02/09 Habbal_Astro110-01 Lecture 14 2

Atomic Terminology • Atomic Number – number of protons in nucleus • Atomic Mass Number – number of protons + neutrons 20/02/09 Habbal_Astro110-01 Lecture 14 3

Atomic Terminology (cont’d) • Isotope: – same # of protons but different # of neutrons ( 4 He, 3 He) • Molecules: - two or more atoms (H 2 O, CO 2 ) 20/02/09 Habbal_Astro110-01 Lecture 14 4

How do light and matter interact? • Emission • Absorption • Transmission: – Transparent objects transmit light. – Opaque objects block (absorb) light. • Reflection – Scattering = random reflection 20/02/09 Habbal_Astro110-01 Lecture 14 5

Reflection and Scattering Movie screen scatters light in Mirror reflects light all directions in a particular direction 20/02/09 Habbal_Astro110-01 Lecture 14 6

Interaction of Light with Matter Interaction between light and matter determines the appearance of everything around us. 20/02/09 Habbal_Astro110-01 Lecture 14 7

Probing Matter through Light: The spectrum 20/02/09 Habbal_Astro110-01 Lecture 14 8

Different ways of visualizing a spectrum 20/02/09 Habbal_Astro110-01 Lecture 14 9

Three Types of Spectra: Kirchhof’s Laws Continuous Emission Absorption 20/02/09 Habbal_Astro110-01 Lecture 14 10

Spectra of astrophysical objects are usually combinations of these three basic types Continuous Spectrum Absorption Line Spectrum Emission Line Spectrum 20/02/09 Habbal_Astro110-01 Lecture 14 11

Continuous Spectrum • The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption. 20/02/09 Habbal_Astro110-01 Lecture 14 12

Emission Line Spectrum • A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines. 20/02/09 Habbal_Astro110-01 Lecture 14 13

Absorption Line Spectrum • A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum. 20/02/09 Habbal_Astro110-01 Lecture 14 14

How does light tell us what things are made of? 20/02/09 Habbal_Astro110-01 Lecture 14 15

Recall the parts of an atom Electron cloud Nucleus Atom proton neutron 20/02/09 Habbal_Astro110-01 Lecture 14 16

Chemical Fingerprints  Each type of atom has a unique spectral fingerprint 20/02/09 Habbal_Astro110-01 Lecture 14 17

Production of Emission Lines 20/02/09 Habbal_Astro110-01 Lecture 14 18

Chemical Fingerprints Energy Levels Each type of atom has a unique set of energy levels. Each transition corresponds to a unique photon energy and Energy levels of hydrogen frequency (or wavelength). E = h x f = h / λ 20/02/09 Habbal_Astro110-01 Lecture 14 19

Transitions yielding emission lines • Downward transitions produce a unique pattern of emission lines. 20/02/09 Habbal_Astro110-01 Lecture 14 20

Transitions yielding absorption lines • Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths. 20/02/09 Habbal_Astro110-01 Lecture 14 21

Production of Absorption Lines 20/02/09 Habbal_Astro110-01 Lecture 14 22

 Chemical fingerprints in a spectrum indicate which kinds of atoms are present 20/02/09 Habbal_Astro110-01 Lecture 14 23

Example: the Solar Spectrum 20/02/09 Habbal_Astro110-01 Lecture 14 24

How does light tell us the temperatures of planets and stars? 20/02/09 Habbal_Astro110-01 Lecture 14 25

Thermal Radiation • Nearly all large or dense objects emit thermal radiation, including stars, planets, and you. • An object’s thermal radiation spectrum depends on only one property: its temperature. 20/02/09 Habbal_Astro110-01 Lecture 14 26

Properties of Thermal Radiation 1. Hotter objects emit more light at all frequencies per unit area. 2. Hotter objects emit photons with a higher average energy. 20/02/09 Habbal_Astro110-01 Lecture 14 27

Wien’s Law λ max = 2.9 10 6 / T (K) 20/02/09 Habbal_Astro110-01 Lecture 14 28