Infrared

The frequency range from $3 \times 10^{11} \, {\rm Hz}$ to $\sim 4 \times 10^{14} \, {\rm Hz}$ is referred to as the infrared band (IR). The frequency is just below that of red light. The IR band is again subdivided as follows

intermediate IR   3000 6000 nm 
near IR 		 780 		  -3000nm 


intermediate IR 		 3000 		 - 6000 nm 


far IR  IR 		 6000 		 - 15000 nm 


extreme IR 		 15,000 nm   - 1mm 

The division are quite loose and it varies.

There are many molecular vibrational and rotational transitions which produce radiation in the IR. Vibrational transitions of $CO_2$ and $H_2O$ fall in the energy $~ 0.2-0.8$ eV range. The energy of a photon is calculated by multiplying its frequency with Planck constant $h$ and converting the Joules in electron-volts(eV)( 1eV $=1.6\times % 10^{-19}$ Joules). Usually small wavelength (or the large frequency) bands of the electromagnetic radiation are distinguished by their energies. Further, the black body radiation from hot objects omit copious amounts of IR. as the spectrum typically peaks in this bond .

For example the black body radiation from human beings peaks at around $10,000 \, {\rm nm m}$ asking the IR suitable for ``night vision". Some snakes sense their preys using infrared vision in night. Stars like the sun and incandescent lamps emit copiously in the IR. In fact an incandescent lamp radiates away more than 50% of its energy in the IR.

IR spy satellites are used to monitor events of sudden heat generation indicating a rocket launch or possibly nuclear explosion.

Much of fiber optical communication also works in the IR.