Sentence:  This involves the application of a strong magnetic field to the radical vapor , which shifts the low-frequency spectra to a conveniently high <WSD>microwave</WSD> range , where they may be measured with optimum sensitivity . 
Meaning of the word marked with <WSD> tags:  a short electromagnetic wave (longer than infrared but shorter than radio waves); used for radar and microwave ovens and for transmitting telephone, facsimile, video and data