The Electromagnetic Spectrum

1. The Electromagnetic Spectrum PHYSICS 1 • Somnath Bharadwaj and Pratik Khastgir, Department of Physics and Meteorology, IIT Kharagpur, 721 302 India http://www.cts.iitkgp.ernet.in/~phy1/

2. Radio wave Less than 1 GHz

3. Microwave 1 GHz to 3  1011 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 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  1011 Hz to 4  1014 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  1014 Hz to 7.69  1014 Hz Mainly atomic transitions – outer levels Hot bodies ~5000K

14. Ultraviolet 8  1014 Hz to 3  1016 Hz Enough energy to ionize atoms in upper atmosphere Is harmful – absorbed by O3 in upper atmosphere Produced in energetic atomic transitions

15. X-ray 2.4  1016 Hz to 5  1019 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  1019 Hz Produced in nuclear transitions Electron-positron annihilation Easy to detect – ionizes gas