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By virtue, humans eye can only see EM wave in the visible light region. From my understanding of 'why we see things', it is because light reflect off an object and the lights' diffraction patterns that enter our eyes determine our ability to resolve it.

My textbook then says "the wavelength of visible light is too long for light to interact effectively with individual atoms and molecules."

However, what exactly does "interact effectively" mean? Does it mean light don't reflect from individual atoms or molecules? If so why would the wavelength of a wave affect its ability to reflect off a small thing e.g. an atom?

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the wavelength of visible light is too long for light to interact effectively with individual atoms and molecules

That's some extremely clumsy wording on the part of your text. The wavelength of visible light is indeed much, much longer than the characteristic sizes of atoms and molecules (half a micron to half an angstrom) but that doesn't mean that they cannot interact.

It does mean that as far as the light is concerned both atoms and molecules are point sources, so the light won't be able to interact with the specific shape of the molecule or image it.

However, if the frequency of the light is correctly tuned to the frequencies at which the molecule can respond (sharp levels in gas phase, broader bands in condensed matter), then the light will induce charge oscillations in the molecule and those will feed back to the EM field as scattered or reflected light.

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