microwave

Microwave is electromagnetic radiation (EMR) with wavelength in the range of 1mm to 1m (frequency 300MHz to 300GHz). It comprises the shortest radio waves, adjacent to infrared.

The term microwave astronomy is occasionally used and millimeter astronomy spans the border between microwave and infrared. The atmosphere blocks much microwave EMR from reaching Earth, and some space observatories have been used. However, the long-wavelength end of microwave is within the radio atmospheric window, narrow windows allow some shorter microwaves through, and high altitudes are used for microwave observatories to sense more signal.

Targets of study include the cosmic microwave background which has motivated three space observatories, as well as the 21cm line.