lecture7_ch5a - Review Ch 4: On Orbits • Orbital energy...

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Unformatted text preview: Review Ch 4: On Orbits • Orbital energy is conserved. • Orbits cannot change spontaneously. • An object’s orbit can only change if it gains or loses energy: – Gravitational encounter. – Atmospheric drag. } Light: The Cosmic Messenger How to use light to learn what a star is made out of and how hot it is Examples What is light? Solar Spectrum A Spectrum Spectra Intro to spectroscopy 1 A Spectrum Light is a Wave Wavelength: distance between peaks Plot of light intensity vs. wavelength Light is a Wave Frequency: the number of times per second the wave bobs up & down (Hertz) Light is a Wave “Electromagnetic” wave: The speed of light in a vacuum is a constant: Light interacts with electric charges and magnets c=300,000 km/sec Wavelength x Frequency = c The Electromagnetic Spectrum Light is a Particle Photons: particles of light. wavelength, frequency, and energy. Bluer light, shorter wavelength, higher frequency, & higher energy. Visible Gamma X-rays Rays UV IR Radio Redder light, longer wavelength, lower frequency, & lower energy. Electromagnetic_spectrum 2 What is matter? Elements and Atomic Structure • Elements: examples H, He, C, N, O. • Elements are made of atoms. • Atoms – Protons (+) & Neutrons (0) = Nucleus – Electrons (-) Atomic Terms • Atomic Number: # of protons in nucleus • Atomic Mass Number: # of protons and neutrons in nucleus (4 He) • Isotope: Same atomic #, different mass #. (4He and 3 He) Molecules: two or more bound atoms (H2 O, CO2 ) Ion: an charged atom (# of protons not = # of electrons) How do light and matter interact? Matter How do light & matter interact? • • • • Light Emission The Sun, electric lights, you Absorption Anything opaque, you Transmission Windows, glass filters Reflection or Scattering Transparent vs. Opaque • Transparent: transmits light • Opaque: blocks (absorbs or reflects) light You, movie screen, walls, mirror 3 What is color? An object reflects some wavelengths of light and absorbs, transmits, and emits others. What we see depends on our eyes & brain. Why is the rose red? a) b) c) d) The rose absorbs most light. The rose transmits some light. The rose emits infrared light. The rose reflects red light. HINT: all of these statements are true. What types of light can we observe? Types of Light Spectra 1. 2. 3. 4. Thermal radiation (continuous rainbow) Absorption lines (dark lines on rainbow) Emission lines (bright lines) Combinations of the above How does light tell us what things are made of? (Production of absorption lines anim) 4 Electrons and Light Interaction of Light with Matter Only photons whose energies match the change in electron energy levels are emitted or absorbed. Hydrogen Chemical Fingerprints Hydrogen Energy Levels • Only specific electron energies allowed. • Absorption or Emission Solar Spectrum • Every atom, ion, and molecule has a unique spectral “fingerprint” Composition mystery gas A Spectrum How does light tell us the temperatures of planets and stars? 5 Thermal Radiation Thermal Radiation • All everyday objects emit it (including you) • Only diffuse, almost transparent objects don’t emit it. • The spectrum depends on only one property: temperature Temperature (repeat) Kelvins (K): temperature unit T (in K) is directly proportional to the random Kinetic Energy of the gas particles “Absolute zero” means zero motion Thermal Radiation Light per area Wienʼs law animation Two 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. Summary (Ch 5, part 1) • Properties of light: order of the EM spectrum, relationships between wavelength, frequency, energy • Properties of matter and elements and atoms • How light and matter interact. • How to identify elements with emission or absorption lines. • What thermal radiation is and how to estimate the temperature from the thermal radiation. 6 ...
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