High-frequency light is useful. The higher the frequency of light, the shorter its wavelength – and the shorter the wavelength, the smaller the objects and details the light can be used to see.
So violet light can show you smaller details than red light, for example, because it has a shorter wavelength. But to see really, really small things – down to the scale of billionths of a metre, thousands of times less than the width of a human hair – to see those things, you need extreme ultraviolet light (and a good microscope).
