Spectrometer Breakout Board with AS72651, AS72652 and AS72653 Sensors

The light our eyes see (that is, the light our brain interprets) is only a portion of what the sun sends us at every moment, and it travels together with many other electromagnetic waves covering a very wide range of frequencies. This whole set of frequencies is called the spectrum.

Our eye perceives only a part of the spectrum of light radiation that makes up sunlight, and that part is called the visible spectrum, or visible light. As it happens, the visible spectrum matches the frequencies with the greatest intensity in the sunlight that reaches the Earth; and this is no coincidence, but the result of the evolution of the human eye, which over the millennia has become tuned to exactly the most important part of the light spectrum striking our planet.

When light strikes an object that does not absorb it completely (otherwise we would be dealing with what is known as a black body), part of it comes back: this optical phenomenon is commonly known as reflection. If that reflected light reaches our eye and carries a frequency within the visible spectrum, we see the object in the matching colour.

So when we see a green leaf, that leaf has in fact absorbed all the other colours and reflected only the green component, that is a wave with a wavelength of around 520nm.

Reflected light carries more than colour alone: if we light a material with white light (the sum of every visible frequency) and it reflects only violet light, it means it has absorbed all the other frequencies.

The set of frequencies an object captures is called its absorption spectrum and, since every material has a known absorption spectrum, that spectrum works as an identity code for the material itself.

The idea matters enough to have earned a branch of physics that studies the interaction between light and matter: spectrometry, and the instrument used to measure that interaction is called a spectrometer.