# Making Sense of Light: The Use of Optical Spectroscopy Techniques in Plant Sciences and Agriculture
## 2 Historical Perspective  
Although spectral phenomena such as the rainbow have been known to humans since ancient times, spectroscopy as a science emerged in the 17 th century. At that time, experimenting with sunlight and prisms, Sir [PERSON] first demonstrated that white light is composed of seven visible colors [13]. Soon after, the investigations were extended to light from stars and flames, and the first signs that the spectra are related to the chemical composition of the burning sample were observed. Afterward, spectroscopy started to develop more intensively, accompanying the First Industrial Revolution. The foundations of the wave theory of light and its interference were formed. The pioneer Newtonian setup, built around a small hole and a prism, was improved by using a slit for the light beam formation and, later, a diffraction grating [14; 15; 16] for the light dispersion. This enabled [PERSON], [PERSON] and, later, [PERSON] to observe and describe the position (wavelengths) of about a thousand thin dark lines of the solar spectrum [17; 18; 19], presently known as the Fraunhofer lines. For the accurate measurements of the angular position of the lines, a two-prism setup and a telescope were mounted on a precise rotation stage to observe the spectrum, turning the optical arrangement into what we today call a spectroscope (see [20] for more details). The establishment of a quantitative wavelength scale served as the basis for a consistent cooperative examination and classification of spectra observed in different laboratories, from diverse sources, and using various instruments. By examining starlight and flames with the spectroscope, it was found that they also can emit bright spectral lines [14]. Further critical development of spectroscopy was made by [PERSON] and [PERSON], who reinforced the initial assumption of the relationship between spectra and chemical composition with solid experimental data [21]. By the beginning of the 21 st century, the ever-increasing progress in the manufacture of optical components--together with the development of each time more sensitive and accurate photodetection techniques and devices--formed the foundations of spectroscopy as we know it today. The rhythm of this progress in the field of specialized instrumentation, such as fiber optics, gratings, and CCD, is well tracked in a number of fast-paced online publications [22]. It grew into a broad and one of the most important fields of physics and chemistry, going beyond the framework of optics and inorganic chemical analysis. Since then, spectroscopy has evolved using different strategic physical concepts, including such exotic ones as acoustic spectroscopy and dynamic mechanical analysis [23]. The study of radiation remains the predominant area of activity; however, radiation has been considered in its broadest sense, giving rise to microwave, terahertz, traditional optical (i.e., far/near/mid-infrared, visible-light, and ultraviolet), X-ray, and gamma spectroscopy. Spectroscopicmethods can also be classified in accordance with the nature of the interactions between radiation and materials [24]. Here, along with the simplest elastic-scattering and reflection techniques, one can distinguish several others, based on more complex light-matter interactions, including, absorption, emission, Raman, Brioullin, coherent and nuclear spectroscopy. The interested reader is referred to the extensive treatise [25], and other vast literature on the subject. In plant sciences, the spectroscopic methods are applied to the solution of a wide range of problems, from plant tissues and composition analysis to the detection and characterization of motile species and biofilms. In view of the great diversity of these problems, the plant scientists use a wide arsenal of spectroscopic methods covering nearly all spectral ranges of electromagnetic radiation and various types of its interaction with matter [22; 24], from simple reflectance up to laser ablation inductively coupled plasma mass spectrometry.