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Light and Matter

Light and Matter. Astronomy: The Science of Seeing. How do you do Astronomy?. How do Chemists do Chemistry? Make solutions, mix chemicals … How do Biologists do Biology? Breed fruit flies, (and whatever else biologists do). They devise and conduct experiments in their labs.

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Light and Matter

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  1. Light and Matter Astronomy: The Science of Seeing

  2. How do you do Astronomy? • How do Chemists do Chemistry? • Make solutions, mix chemicals … • How do Biologists do Biology? • Breed fruit flies, (and whatever else biologists do). • They devise and conduct experiments in their labs. • But how do you do that for astronomy?

  3. Light • Astronomy is a “passive” science. • We can’t (yet) go to the stars or other galaxies. • The Universe must come to us. • We rely on light exclusively!

  4. But what does it look like from the back?

  5. What you see is all you get! • So you need to squeeze EVERY last drop of information out of the light we get. • This semester we’ll see how we can use light to: • Weigh a planet. • Take a star’s temperature. • Tell what’s in the center of a star a thousand light-years away. • Tell what our Galaxy look like from the outside.

  6. Tonight We’ll Learn: • What is light? • How is it produced? • Continuum Blackbody radiation (this lecture) • Line radiation (next lecture) • How do light and matter affect each other? • How are we able to learn about the Universe from this light?

  7. The “Visible” Spectrum • When you think of “light”, what do you think of?

  8. What is Light? • Light is an electromagnetic wave. • Propagates through a vacuum. • Travels at the speed of light (a CONSTANT): c = 3 x 1010 cm/s • The wavelength (l) and frequency (n) are related: c = ln • The energy is inversely proportional to the wavelength (where h is a constant): E = hn E = hc/l

  9. Radio Optical and infrared g-ray UV X-ray

  10. What’s the Wavelength? • Arrow 93.1 FM • 93.1 MHz (Mega Hertz) = 93.1 x 106 cycles/sec c = ln 3 x 1010 cm/sec = l x 93.1 x 106 cycles/sec l = (3 x 1010 cm/sec)/(93.1 x 106 cycles/sec) l = 322 cm =3.22 m How big is your radio antenna?

  11. To Sum Up… • Radio waves, microwaves, rainbows, UV waves, x-rays, etc are ALL forms of electromagnetic waves. • They ALL travel through space at the speed of light. c • The higher the frequency, the shorter the wavelength. c = ln • The higher the frequency, the more energetic the wave. E = hn

  12. The Sun • Most objects emit light at more than one wavelength, thus producing a spectrum. • Why? • One Reason: Temperature!

  13. Matter • Atoms consist of a positively charged nucleus surrounded by a negatively charged cloud of one or more electrons.

  14. Atoms in Motion • Everything is composed of atoms which are constantly in motion.

  15. Temperature • The hotter the object, the faster the average motion of the atoms. COOLER HOTTER

  16. Atoms and Light • As atoms move they collide (interact, accelerate). • Collisions give off energy. • But light IS energy. E = hn

  17. Light and Temperature • The hotter the object the faster the average atom and the more energetic the average collision. • The faster the atoms the more collisions there are.

  18. HOT COLD

  19. Energy and Intensity • The more energetic the average collision the bluer the average light that is given off. • Since E = hn • The more collisions that occur the more light that is given off.

  20. Blackbody Laws • Put another way: Wien’s Law for peak wavelength (lpeak): lpeakis proportional to 1/T Stefan-Boltzmann Law for total Flux (F): F is proportional to T4

  21. lpeaka 1/T F a T4 Hottest Hotter Hot Graphically

  22. IR Result • HOT toasters are BRIGHTER than cool toasters. • HOT toasters are BLUER than cool toasters. • What is the peak wavelength for something at room temperature (a cool toaster, or a cool person)? lpeak= k* 1/T lpeak= (2.898 x 10-3 m/K) * 1/ 300 K lpeak= 9.6 x 10-6 m

  23. Blackbody Radiation • Light given off by an object due solely to its temperature. • Don’t confuse with reflected light: • Buses are yellow not because they are hot enough to emit visible radiation but rather they reflect the yellow light given off by the Sun. • What kinds of blackbody radiation do we see in our everyday life?

  24. The IR World • Since everyday objects (at everyday temperatures) emit blackbody radiation in the IR, this is why we perceive IR as HEAT. http://www.x20.org/library/thermal/blackbody.htm

  25. The IR Universe • Everyday things that are hot radiate in the IR: • Dust – There are interstellar clouds of dust. Orion – by IRAS

  26. The IR Universe Io from IRTF. • Molten Rock – There are lava flows on a moon of Jupiter. Orion – by IRAS

  27. The IR Universe • Differences in composition lead to differences in temperature. Orion – by IRAS The Moon in eclipse.

  28. The Greenhouse Effect • Why is my car hot on a summer day? • At T = 6000 K, the Sun radiates mostly visible light. Windshield is transparent to visible light. • Car seat absorbs this visible light and warms up to 400 K. • At T = 400 K, my seat radiates mostly at longer wavelengths in the IR. Windshield is opaque in the IR. • Result: Energy is TRAPPED inside the car!

  29. Venus and Earth • Certain gases act the same way as your windshield: Carbon Dioxide (CO2). • Venus – Runaway greenhouse effect. • Earth – Could that happen here?

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