1. The Electromagnetic Spectrum
PHYSICS 1
The Electromagnetic Spectrum
Somnath Bharadwaj and Pratik Khastgir, Department of Physics and Meteorology, IIT Kharagpur, India

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
Frequency MHz
153
235
325
610
1420 z
8.3
5.0
3.4
1.3
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 nm
Far IR nm
Extreme IR nm – 1 mm
Human body peaks at 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. Credit: J. Sanders, A. Fabian,
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