Our notion of an image is inseparable from how we perceive the world through our senses and, in this particular case, from our visual system. The brain has the delicate task of interpreting the information collected by the eye and transmitted through the optic nerve. As Plato had already observed (in The Republic, VI), it is the sun, the "image of the Good"—that is, light—that enables our eyes to distinguish the colors of things. Without light, there can be no sensation of color, and therefore no image. The concept of color is thus inseparable from the phenomenon of light (see our feature "Mathematics and Light," Tangente 164, 2015), and this relationship lays the groundwork for a scientific description through physics and optics.
Toward a model of light -----------------------------------
In physics, the term "light" refers to a particular band of electromagnetic radiation, each characterized by a certain range of wavelengths. Our eyes perceive some of it: this is the light we call visible, with wavelengths ranging approximately from 400 nanometers (violet) to 700 nm (red); one nanometer, or nm, equals 10–9 meters. A more or less uniform mixture of frequencies within this interval produces what our brain interprets as white light. This window corresponds roughly to the wavelengths most readily transmitted by Earth's atmosphere, which fits perfectly with theories of evolution. Beyond this spectrum lie ultraviolet radiation on one side (from 10 to 400 nm) and infrared radiation on the other (from 700 nm to 3 mm). Wavelengths between 3 mm and 30 cm are those of microwaves, mobile phones, and radar. Longer wavelengths correspond to radio waves.
Light can also be represented in terms of particles. In this formal framework, every electromagnetic wave is associated with a photon—the corresponding quantum of energy—to which certain characteristics of elementary particles are ascribed. In modern physics, the interaction between two charged particles is modeled as an exchange of photons. Some light phenomena can be described using electromagnetic waves, while others can be described by taking the particle nature into account. Quantum theory has provided a remarkably successful formal account of this astonishing duality (see Les Équations de la physique moderne (The Equations of Modern Physics), Bibliothèque Tangente 71, 2020).
From experiment to theory ----------------------------