Light and Temperature

1. Light and Temperature Astronomy: The Science of Seeing

2. Goals • What is light? • What are the types of light? • Where does the light we see come from? • Understanding the light of heat. • On a sunny day: • Why does it seem hotter wearing a black T-shirt versus a white one? • Why are they different?

3. What is Light? • Light is a wave of energy. • Moves 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 of light is: E = hn = hc/l

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

6. To Sum Up… • Radio waves, microwaves, rainbows, UV waves, x-rays, etc are ALL forms of light (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 energy. E = hn

7. Concept Test • Yellow light has a longer wavelength than blue light. Therefore yellow light has: • A higher frequency and higher energy than blue light. • A higher energy and lower frequency than blue light. • A lower frequency and lower energy than blue light. • A lower energy and higher frequency than blue light. • None of the above.

8. Emission lines Absorption lines Continuum A Spectrum • A spectrum = the amount of light given off by an object at a range of wavelengths.

9. Three Reasons All objects do one or more: • Reflect light because of color or smoothness (same as scatter) • Emit light because of their temperature (thermal radiation) • Emit or absorb light because of their composition (spectral lines) A person, house, or the Moon: reflects visible light, and because each is warm, emits infrared light.

10. Temperature and Light • Warm objects emit light. • Thermal radiation

11. Kelvin Temperature • Kelvin: an absolute scale. • Kelvin is Celsius + 273 degrees. • Water freezes: 0 C  273 K • Water Boils: 100 C  373 K • Room Temp: 80 F  27 C  300 K • Surface Sun: 5800 K

12. Thermal Radiation Laws • Hotter is bluer. • (peak at shorter wavelength) • Hotter is brighter. • (More intense at all wavelengths)

13. visible range Star A Energy Output per second Star B V I B G Y O R Wavelength Concept Test Which of the two stars (A or B) is at a higher temperature? a. Star A b. Star B c. The two stars have the same temperature. d. It is not possible to infer this relationship.

14. visible range Star A Energy Output per second Star B V I B G Y O R Wavelength Concept Test Which of the following best describes how Star A would appear compared to Star B? a. Star A would appear more red than Star B. b. Both stars would appear more red than blue. c. Both stars would appear more blue than red. d. Star A would appear more blue than Star B. e. None of the above.

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

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

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

18. 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. HOT COLD

19. Energy and Intensity • The more energetic the average collision the bluer the average light that is given off. • Since E = hc/l • The more collisions that occur the more light that is given off per surface area. • 1. Hotter is bluer. • (peak at shorter wavelength) • 2. Hotter is brighter. • (more intense at all wavelengths)

20. Thermal Radiation Laws • Hotter is bluer. • (peak at shorter wavelength) • Hotter is brighter. • (More intense at all wavelengths)

21. Thermal versus Reflection • Thermal radiation is light given off because of an object’s 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 thermal radiation do we see in our everyday life?

22. The IR World • Everyday objects (at everyday temperatures) emit thermal radiation in the IR, this is why we equate IR with HEAT. http://www.x20.org/library/thermal/blackbody.htm

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

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

25. The Moon in eclipse. R. Gendler The IR Universe • In eclipse, there is no reflected light. • Only thermal radiation. • Differences in composition lead to differences in temperature. Orion – by IRAS

26. Color Why’s • Why is that shirt blue? • It’s hot. • It’s cool. • It’s hotter than a red shirt. • It’s reflecting blue light. • It’s absorbing blue light, thus making it blue.

27. Color Why’s • Why is the Sun yellow? • It’s cooler than a blue flower. • It’s hotter than a person. • It’s reflecting yellow light. • It’s absorbing yellow light, thus making it yellow. • None of the above.

28. Color Why’s • Why is this paper white? • It’s temperature. • It’s hotter than the sun but cooler than a blue shirt. • It reflects only white wavelengths of light, and absorbs the rest. • It absorbs white light, thus making it white. • It reflects all visible light equally.

29. Color Why’s • Why is the light filament orange? • It is hotter than a person, but cooler than the sun. • It is cooler than a blue shirt. • It reflects orange light. • It absorbs orange light, thus making it orange. • None of the above.

30. Color Why’s • Why is Mars red? • It’s hot. • It’s cool. • It’s reflecting red light from the Sun. • It’s absorbing red light from the Sun. • None of the above.

31. Homework #8 • For Monday 22-Sept: Read B5.3 – 5.5 • Do B5: Problems 35, 38,57 • Consider the four stars (A,B,C,D) of Problem 46 in Chapter B5. Arrange them by speed relative to us (starting with the fastest moving towards us and ending with fastest away). • A, B, C, D • B, A, D, C • D, C, B, A • C, D, A, B • None of the above Don’t forget Topic of Confusion.