Blackbody Radiation and Spectroscopy

1. Blackbody Radiation and Spectroscopy Plus … Telescopes and Imaging Telescopes and Imaging 2/1/07

2. Announcements • Second homework is due on Tuesday • Next-week’s reading assignment • Sections 7-1, 7-2, 7-4, 7-5, and 7-6 (pp. 146-160) Telescopes and Imaging 2/1/07

3. Today’s topics • Origin of light • Blackbody radiation • Wien’s Law • Stefan-Boltzman Law • Light as a particle • Emission and absorption spectra and Kirchoff’s Laws • Telescopes -- basics Telescopes and Imaging 2/1/07

4. The Intensity of Light decreases with distance from the source and obeys the Inverse Square Law Telescopes and Imaging 2/1/07

5. Origin of Light • Atomic, molecular emissions • Every atom, molecule has a characteristic spectrum (like a fingerprint) • Caused by transitions from one energy level to another • Continuum • Everything that’s heated glows • Color depends on temperature Telescopes and Imaging 2/1/07

6. Temperature Scales • Kelvin • used by most astronomers and planetary scientists • 0 K is “absolute” zero • Kelvin  Celsius TC = TK - 273 • Celsius  Fahrenheit TF = (9/5)TC + 32 • Fahrenheit  CelsiusTC = (5/9)(TF-32) Telescopes and Imaging 2/1/07

7. Blackbody radiation • A blackbody is a hypothetical object that is a perfect absorber of electromagnetic radiation at all wavelengths • The Sun closely approximates the behavior of blackbodies, as do other hot, dense objects • The intensities of radiation emitted at various wavelengths by a blackbody at a given temperature are shown by a blackbody curve Telescopes and Imaging 2/1/07

8. The Sun is like a Blackbody Telescopes and Imaging 2/1/07

9. Wien’s Law • The higher the temperature, the smaller the wavelength of maximum emission Example: A heated metal rod will start to glow red, then get brighter and glow yellow, then get brighter still and turn blue and then white Telescopes and Imaging 2/1/07

10. Wien’s Law Wien’s law states that the dominant wavelength at which a blackbody emits electromagnetic radiation is inversely proportional to the Kelvin temperature of the object Telescopes and Imaging 2/1/07

11. Stefan-Boltzmann Law • The Stefan-Boltzmann law states that a blackbody radiates electromagnetic waves with a total energy flux Fdirectly proportional to the fourth power of the Kelvin temperature Tof the object: F= T 4 σ is called the Stefan-Boltzmann constant This law can be used to determine the temperature of the Sun, starting with a measurement of the amount of light arriving at Earth. (see Box 5-2 of the textbook) Telescopes and Imaging 2/1/07

12. Light also behaves like a particle • Max Planck was able to derive the blackbody spectrum by assuming that light was made up of tiny, discrete packets of energy – called photons • Energy of a photon (light) with a wavelength, λ ħ = Planck’s constant = 6.625 x 10-34 J • s Telescopes and Imaging 2/1/07

13. Photoelectric effect • When UV light strikes a metal plate, electrons are emitted by the metal and can be detected • When the plate is illuminated by visible light, no electrons are emitted. • In the light-as-a-particle picture, this can be understood ! • UV light has a shorter wavelength and a higher energy compared to visible Telescopes and Imaging 2/1/07

14. The Modern View of Atomic Structure • Protons, Neutrons, Electrons • Size • About 10-10 m (1 Å – or 1 Angstrom) • Nucleus is only 10-14 m !! • Mass • Protons, Neutrons ~10-27 kg • Electrons ~10-31 kg • The nucleus has most of the mass, but is less than 0.03% by volume of the entire atom! Telescopes and Imaging 2/1/07

15. What do Atoms have to do with Planetary Science? • Spectroscopy! • Emission and Absorption Lines • Each element emits/absorbs at a specific wavelength that is unique to that element • This fact can be used to infer the composition of a body or its atmosphere Telescopes and Imaging 2/1/07

16. Kirchoff’s Laws • A hot opaque body (blackbody) produces a smooth continuous spectrum • Example: stars • A cool transparent gas in front of a source of a continuous spectrum produces an absorption-line spectrum • Example – planetary atmospheres, solar photosphere and chromosphere • A hot transparent gas radiates an emission-line spectrum (against a dark background) • Example: the solar corona Telescopes and Imaging 2/1/07

17. Absorption lines in the Solar Spectrum Indicates the presence of Iron in the Sun Telescopes and Imaging 2/1/07

18. Light Scattering:The reason the sky is blue (on Earth!) Telescopes and Imaging 2/1/07

19. Look how dark it is in the shadow of the Apollo 11 lander On Earth (Tucson Barrio), we can see just fine in the shadows Telescopes and Imaging 2/1/07

20. The “Green Flash” Telescopes and Imaging 2/1/07

21. Telescopes and Astrophotography • Basic telescope types and how they work • Magnification and Resolution • Atmospheric Turbulence • Hubble • Adaptive Optics • Basics of Astrophotography Telescopes and Imaging 2/1/07

22. Telescopes and Imaging 2/1/07

23. Basic Telescope Types • Refractor • Reflector • Newtonian  • Schmidt-Cassegrain (adjacent photo) Telescopes and Imaging 2/1/07

24. Magnification • The amount of magnification depends on the focal length of the primary and the eyepiece Telescopes and Imaging 2/1/07