The Electromagnetic Spectrum

Radio wave Less than 1 GHz

Microwave 1 GHz to 3  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 z 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  Hz to 4  Hz Near IR nm Intermediate IR nm Far IR nm Extreme IR 15000nm – 1 mm Human body peaks at nm

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

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

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

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

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