wavelength
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Watch ripples spread across a pond: the distance from the crest of one ripple to the next is its wavelength. Light travels as waves too, and that same crest-to-crest distance decides what colour the light is — and which substances will absorb it.
Wavelength is the distance between successive peaks of a wave, written with the Greek letter lambda and measured in nanometres for light. In the optical region, short wavelengths are blue and ultraviolet while long ones are red and infrared; each substance absorbs most strongly at its own characteristic wavelengths.
Choosing the right wavelength is the first decision in any spectrophotometric method. Analysts pick the wavelength of maximum absorption, because there the signal is strongest, the response is flattest against small wavelength errors, and sensitivity is highest — giving the most reliable link between absorbance and concentration.
A copper-ammonia complex is deep blue because it absorbs orange-red light near 600 nm; so an analyst measures it at 600 nm, where its absorbance peaks.
A substance is measured at the wavelength it absorbs most strongly.
The colour we see is the leftover light — the complement of what was absorbed. A solution looks blue precisely because it removed the orange wavelengths.