# Relationship between sensors and materials in terms of electromagnetic spectrum

The following text is from Digital Image Processing(Gonzalez and Woods)[3ed, Ch 2.2,p46]

If a sensor can detect energy radiated by a band of electromagnetic spectrum, we can image events in that band. But the wavelength of electromagnetic wave required to “see” an object must be same size or smaller than the object.For example, a water molecule has diameter $10^{-10}$ m. To study water molecules, a source capable of emitting in far ultraviolet or soft X-ray region is required.

For the water molecule example, why do the authors refer to a source emitting ? As an analogy, for X-rays, the bones absorb waves in X-ray region and this renders them opaque on the film. Therefore, to image water molecules, a source which emits far ultraviolet/soft X-rays is needed. Passing this through water would somehow let us image water molecules. Is that what they mean ?

But the wavelength of electromagnetic wave required to “see” an object must be same size or smaller than the object.

Essentially, the waves are unaffected (at least not enough to be detected) by objects that are smaller than the wavelength. Since we look at the distortion in the waveform to image objects, no distortion would mean an object is undetectable.

Imagine you are walking down a rocky path. Small rocks and pebbles would not change your gait at all, so they would be undetectable to you. Large stones, however, would force you to change your step, making them detectable. If you were much smaller and took shorter steps (or a shorter wavelength), then these small objects would become detectable to you.