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

Radio wave Less than 1 GHz

Microwave 1 GHz to 3  1011 Hz 30 cm to 1 mm

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

The 21 cm HI radiation

Star and Gas Distribution

Radio Interferometric Arrays Frequency MHz 153 235 325 610 1420 z 8.3 5.0 3.4 1.3 32 MHz bands with 128 separate channels

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

Cosmic Microwave Background Radiation T=2.73 K CMBR

Blackbody Radiation

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

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

Visible Light 3.84  1014 Hz to 7.69  1014 Hz Mainly atomic transitions – outer levels Hot bodies ~5000K

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

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

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

Gamma Rays Frequency greater than 5  1019 Hz Produced in nuclear transitions Electron-positron annihilation Easy to detect – ionizes gas