Issue link: https://wardsworld.wardsci.com/i/1304293
Color (continued) + ward ' s science the sensation of seeing any color. The human eye is capable of distinguishing up to 10 million different colors based on the relative responses of the three different cones in the eye. A significant portion of the population, however, has one of several forms of so-called color blindness, which limits abilities in detecting differences in colors readily noticeable to those without the condition. HSL color scheme Most light sources, such as the Sun, a flame, or an incandescent light bulb, contain light of different wavelengths simultane- ously. For instance, sunlight contains all of the colors in the visible spectrum. This can be seen by inserting a prism into a sunbeam, as illustrated in Fig. 4. As a result, the color of a light source is frequently character- ized in terms that specify the central wavelength and range of wavelengths present in the source. The central wavelength is described by the hue; the range of wavelengths (or purity) is described by the saturation; and the overall intensity, or bright- ness, is characterized by the lightness. This is illustrated in Fig. 5 and is known as the HSL color scheme. Fig. 6 illustrates varia- tions in hue, saturation, and lightness for a green patch, which has vertical variations in hue (left panel), saturation (middle panel), and lightness (right panel). The apparent color of an object depends both on the hue and saturation of the light shining on the object, as well as the reflectivity of the object for the different wavelengths present in the light. An object that appears red reflects more red light than other colors in the spectrum. If an object that reflects all colors equally (for example, a white piece of paper) is illuminat- ed by a white light source, then it will appear white. However, if it is illuminated by a blue or green light source, then it will appear blue or green. Conversely, if a red object is illuminated by white light, then it will mostly reflect the red light, while absorbing the other colors/wavelengths of light illuminating it. This is illustrated in Fig. 7, which shows light of many colors in- cident on a red apple with a green leaf. The leaf reflects mostly green light (absorbing or subtracting out the others); therefore, it appears green. The apple reflects mostly red light; therefore, it appears red. Thus, one can describe a colored object in terms of subtract- ing out colors from the incident light except for the color with which we associate the object. Mixing the color of objects that reflect light—such as paint, for example—is therefore often referred to as subtractive color mixing. The three subtractive primary colors are cyan, yellow, and magenta. Each can be defined by subtracting one of the three additive primary colors from white light. Cyan can be made by subtracting the red wavelength portion from white light, yellow by subtracting Fig. 3: Visual response of the human eye to light as a function of wavelength. The response of each of the three cones that define the primary colors are shown together with the overall response. Note that the response curves are not evenly spaced and do not have the same shape. (Credit: Thomas Weinacht) Fig. 4: Sunlight entering a prism from left, which sends wavelengths of light into different directions, allowing one to see the different colors of light present in the sunbeam. (Credit: ktsdesign/Shutterstock)