Purple exists only in our brains

acronym: A word made by combining some of the starting letter or groups of letters from a number of words. For instance, STEM is an acronym for Science, Technology, Engineering and Math. Radar is an acronym for RAdio Detection And Ranging. Even laser is an acronym for Light Amplification by Stimulated Emission of Radiation.

activate: (in biology) To turn on, as with a gene or chemical reaction.

cell: (in biology) The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall.

cones: (in biology) A type of eye cell that is part of the retina inside the back of the eye. These cells can sense red, green or blue light. Recent research has uncovered evidence that many can sense white light — but only white light.

electromagnetic: An adjective referring to light radiation, to magnetism or to both.

electromagnetic radiation: Energy that travels as a wave, including forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.

electromagnetic spectrum: The range of radiation that spans from gamma- and X-rays through visible light and on to radio waves. Each type of radiation within the spectrum typically is classified by its wavelength.

gradient: From the word “grade,” it describes the incline, slope or degree of increase in some measure (such as temperature, pressure or even color) that develops as one moves in time, position or along some scale.

microwaves: An electromagnetic wave with a wavelength shorter than that of normal radio waves but longer than those of infrared radiation (heat) and of visible light.

nanometer: A billionth of a meter. It’s such a small unit that researchers use it as a yardstick for measuring wavelengths of light or distances within molecules. For perspective, an average human hair is about 60,000 nanometers wide.

perception: The state of being aware of something — or the process of becoming aware of something — through use of the senses.

pigment: A material, like the natural colorings in skin, that alter the light reflected off of an object or transmitted through it. The overall color of a pigment typically depends on which wavelengths of visible light it absorbs and which ones it reflects. For example, a red pigment tends to reflect red wavelengths of light very well and typically absorbs other colors. Pigment also is the term for chemicals that manufacturers use to tint paint.

prism: A triangular wedge of glass or another clear substance that can bend the components of white light into a rainbow-like succession of colored bands. (v.) To separate light into its component hues.

radiation: (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

range: The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.

spectrum: (plural: spectra) A range of related things that appear in some order. (in light and energy) The range of electromagnetic radiation types; they span from gamma rays to X rays, ultraviolet light, visible light, infrared energy, microwaves and radio waves.

system: A network of parts that together work to achieve some function. For instance, the blood, vessels and heart are primary components of the human body’s circulatory system. Similarly, trains, platforms, tracks, roadway signals and overpasses are among the potential components of a nation’s railway system. System can even be applied to the processes or ideas that are part of some method or ordered set of procedures for getting a task done.

ultraviolet light: A type of electromagnetic radiation with a wavelength from 10 nanometers to 380 nanometers. The wavelengths are shorter than that of visible light but longer than X-rays.

unique: Something that is unlike anything else; the only one of its kind.

visible light: A type of electromagnetic radiation with wavelengths that range between 380 nanometers (violet) and 740 nanometers (red). Visible light has wavelengths that are shorter than infrared light, microwaves, and radio waves but longer than ultraviolet light, X-rays and gamma rays.

wave: A disturbance or variation that travels through space and matter in a regular, oscillating fashion.wavelength: The distance between one peak and the next in a series of waves, or the distance between one trough and th next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer