Commonly, e.g. if you look up Wikipedia, spectroscopy is described as “the study of the interaction between matter and electromagnetic radiation”. More generally speaking, what you actually want to retrieve by spectroscopy is the response of a system or object to some sort of excitation.
About the similarity of guitar strings and trees
Let us explain the operation of spectroscopy with an example. Think of a guitar as the system or object of interest. When you play it, the guitar (or more specifically its strings) gets excited and delivers a response signal – the music. This signal can be detected by e.g. a microphone and furthermore analyzed by software or other analysis units to represent sound intensity as a function of frequencies contained in the recorded signal – the response or spectrum of your guitar.
The combination of a detection device (the microphone) and the analysis unit (the software) is called a spectrometer. In this respect, your ear is a spectrometer as it detects acoustical waves and (in combination with your brain) converts them into meaningful information – the sound you hear.
Another simple example for a spectrometer is your eye. It converts the interaction of visible electromagnetic radiation (e.g. the sunlight) with any object (e.g. a tree) into a meaningful picture that will be visualized by your brain. Your eye actually is a high-performance spectrometer as it analyzes millions of optical information in a second.
The meaning of wavelength in spectroscopy
However, it is limited to the visible part of the electromagnetic spectrum. What your eye “spectrometer” actually does is to split up the receiving light (which is a superposition of many colors) into the three fundamental colors red, green and blue. This allows your brain (the analysis unit) to identify the colors of the object you are looking at.
Each color is mathematically represented by a wavelength number. The wavelength number is defined as the distance between two consecutive peaks of an electromagnetic wave traveling in space (similar to the distance between peak values of a wave in a lake when you throw a stone into it).
For example, blue color has a wavelength in the range of 450 nm, green around 600 nm and red at roughly 700 nm. Your eye can detect wavelengths in the range from ~400 to ~780 nm, also commonly called the visible range (VIS). If all wavelengths in that range exist at the same time in the radiation your eye receives, you will see white light.
How to identify the “optical fingerprint”
However, there is much more than just the VIS part of the spectrum. In the VIS you get basic information about the color of your object of interest. When you extend the observation range into e.g. the infrared (IR) region above 780 nm, you will be able to detect “fingerprints” of material composition, that means what a material is made of.
This part of the spectrum cannot be seen by your eye but can be made visible by technical spectrometer instruments. Thus, spectroscopy relies on the ability to construct technical instruments which are capable of detecting electromagnetic radiation in the wavelength range of interest with sufficient sensitivity and accuracy, a topic which will be described in a following blog post.