Sentence:  The video signal is amplified and then switched , in synchronism with the three ultraviolet light sources which are sequenced by the rotating mirror so that during one-twentieth of a second only one <WSD>wavelength</WSD> , corresponding to red , green , or blue , is seen . 
Meaning of the word marked with <WSD> tags:  the distance (measured in the direction of propagation) between two points in the same phase in consecutive cycles of a wave