1. The Electromagnetic Spectrum

2. Radio wave Less than 1 GHz

3. Microwave 1 GHz to 3  10 11 Hz 30 cm to 1 mm

4. Space communication Atmosphere is transparent from less than 1 cm to 30 m Also suitable for radio astronomy

5. The 21 cm HI radiation

6. Star and Gas Distribution

7. Radio Interferometric Arrays Frequency MHz1532353256101420 z8.35.03.41.30 32 MHz bands with 128 separate channels

8. HI in Galaxies DDO 210 Source: Begum and Chengalur Dwarf Irregular Galaxy

9. Cosmic Microwave Background Radiation T=2.73 K CMBR

10. Blackbody Radiation

11. Molecular Rotations Water 2.45 GHz used in microwave ovens Excites Rotations of water molecules 50 GHz to 10 THz T-rays

12. Infrared 3  10 11 Hz to 4  10 14 Hz Near IR 760 - 3000 nm Intermediate IR 3000 - 6000 nm Far IR 6000 - 15000 nm Extreme IR 15000nm – 1 mm Human body peaks at 10000 nm

13. Visible Light 3.84  10 14 Hz to 7.69  10 14 Hz Mainly atomic transitions – outer levels Hot bodies ~5000K

14. Ultraviolet 8  10 14 Hz to 3  10 16 Hz Enough energy to ionize atoms in upper atmosphere Is harmful – absorbed by O 3 in upper atmosphere Produced in energetic atomic transitions

15. X-ray 2.4  10 16 Hz to 5  10 19 Hz Energetic electrons incident on a metal Hot astrophysical sources – Black Holes Inner shell transitions in atoms

16. Centaurus Cluster Credit: J. Sanders, A. Fabian,

17. Gamma Rays Frequency greater than 5  10 19 Hz Produced in nuclear transitions Electron-positron annihilation Easy to detect – ionizes gas