Black-Body Radiation

1. Black-Body Radiation • A black-body is a material which absorbs any light falling on it. • A black-body also radiates! • The radiation contains all frequencies, but is peaked at some n which reflects the temperature of the object: n ~ T. • A lot of the heated objects radiate like a blackbody • The sun, a filament….

2. Spectrum of Blackbody radiation at different temperatures

3. Cosmic Microwave Background Radiation • A radiation left over when electrons and nuclei combine into atoms (t=0.3 M years) • It’s a black-body radiation with T=2.7K and wavelength 0.2cm! • 1% of the static in your radio from this “cry of the baby universe”! • First discovered by Pezias and Wilson (1965). Now allows us to see the map of the baby universe (COBE!)

5. How to produce light? • Nuclear Reactions (in the sun) • Chemical reactions (burn stuff, combustion) • Wood, Coal, Oil, Kerosene, Candle, Natural Gas ... • Limelight, gas mantle: inorganic salt (1885) • Electric lamps: (burning electricity) • Arc lamp (charged particles smashing into electrodes) • Filament lamp & florescent lamp 

6. ARC Lamps Broadway New York City 1881

8. Incandescent Filament Lamp (Patented by Edison in 1880) “Be courageous! Whatever setbacks America  has encountered, it has always emerged as a stronger and more prosperous nation.... Be brave as your fathers before you. Have faith and go forward!"

9. Incandescent Filament Lamp • 7% efficiency (others gone to heat). • Higher temp, tungsten filament will melt. • Partially vacuumed, filled with argon and nitrogen, but still the filament evaporate. • In tungsten-halogen lamp, filament is protected with a quartz enclosure.

10. Tungsten Halogen Lamps: http://americanhistory.si.edu/lighting/tech/th.htm

11. Light from discharges • A glass tube is filled with some gas at high pressure. • Electrodes at the end are connected to an alternating current source, which drives the charge first one way and then the other. • The electric fields pull electrons off the electrodes, which then collide with atoms and shake their electrons off.

12. http://www.goodmart.com/facts/light_bulbs/hid_diagram.aspx

13. Different types of HID • Metal HalideMetal Halide lamps offer high efficacy, excellent color rendition, long service life, and good lumen maintenance. They are used often in outdoor applications and in commercial interiors, overhead projector, photoraphy. • SodiumHigh pressure sodium lamps are very energy efficient. Mercury and sodium vapors produce a yellow/orange light with extremely good lumens per watt performance. Although they tend to render colors poorly they have an exceptionally long service life, (up to 40,000 hours). • Xenon gas the latest technology for automobile lighting. You may also hear it called “Xenon” lights.

14. Aurora Borealis (northern lights) Charged particles from the sun strikes the molecules in the upper atmosphere.

16. Fluorescent Lamps • Using discharges to produce ultraviolet radiation (black light) in gas (mercury vapor) • Using phosphors (flouresces) to absorb the UV radiation and produce visible light. • A 40 Watt bulb fluorescent is 4 times more brighter than a 40 Watt incandescent bulb. http://home.howstuffworks.com/fluorescent-lamp.htm

18. Other interesting light Sources • LCD: • http://www.plasma.com/classroom/what_is_tft_lcd.htm • Plasma • http://ask.yahoo.com/ask/20030602.html When an electrical current is applied to a pixel, the gas reacts to form plasma, which in turn produces UV light. The light reacts with the colored phosphors and magically enhances the flat screen to produce higher-contrast ratios and a better, more realistic picture.