Color
“Mere color, unspoiled by meaning, and unallied with definite form, can speak to the soul in a thousand different ways.”
- Oscar Wilde
For a normally sighted person, color is everywhere. Colors can be pleasantly subdued, enhancing relaxation, or loud and calling to us from advertising billboards or magazines. Color entices us to eat, consume, or at least buy. Color likely has helped us to survive as a species. Our (known) contacts with the world and the universe are by way of our five senses. Persons with a normally functioning visual system obtain what is probably the largest amount of information about the world surrounding them from vision, and color experiences are an important outcome of this flow of information. In the past several thousand years, color has blossomed into much more than just a survival and communications tool. We have learned to derive aesthetic pleasure from it by way of crafts, design, and art.
A fabric is dyed with red dye; when painting, we use various colored pigments or draw with various colored crayons or ink pens. The rainbow has four colors, or is it six or seven? In a mirror, we see the colors of objects appearing slightly duller and deeper than in the original. On a winter day toward evening, shadows look deeply blue. We are told that color illustrations in an art book are printed just with four pigments and that all colors on a display screen are “made” from red, blue, and green light-emitting phosphor compounds. To cope with these confusingly varied sources of color, we just disregard them in our everyday languages. An apple is red, the traffic light is red, the rose as seen reflected in a mirror is red, the bar in the bar graph on a tablet display is red, and the paint on the brush is red. All of these varied experiences have something in common: redness. We simply attach the perceived phenomenon to the object without bothering about the source or thinking about the nature of color.
We normally experience color as a result of the interaction between light, materials, and our visual apparatus, eye, and brain. However, there are also means of having color experiences in the dark like pressing against the eyeballs or hitting the temples moderately hard. There are two sets of facts that complicate understanding of the phenomenon of color firstly, many different stimuli can result in an essentially identical color experience and secondly a particular stimulus can result in many different color experiences, usually as the result of changes in illumination and/or surrounding stimuli. The same situation applies to vision in general.
The best definition of color that we can have is " Color: Attribute of visual perception consisting of any combination of chromatic and achromatic content. This attribute can be described by chromatic color names such as yellow or brown, red, pink, green, blue, purple, etc., or by achromatic color names such as white, gray, black, etc., and qualified by bright, dim, light, dark, etc., or by combinations of such names."
Understanding Light
Light consists of a certain range of electromagnetic radiation, which is a convenient name for the as-yet not fully explained phenomenon of energy transport through space. Electromagnetic radiation, depending on its energy content, has different names: X-rays capable of passing through our bodies and, on prolonged exposure, causing serious harm, ultraviolet (UV) radiation that can tan or burn our skin, light that we employ to gain visual information about the world around us, infrared radiation that we experience on our skin as warmth or heat, information transmission waves for radio and television, or electricity transmitted and used as a convenient source of energy. Electromagnetic radiation travels at high speed (the speed of light, about 300,000 km/s). The human eye, our visual sensory organ, is sensitive to a narrow band of electromagnetic radiation, the visible spectrum.
Ways of producing light
Incandescence
Light is normally produced by a glowing body in a process called incandescence; for example, the sun, a burning wax candle, or an electrically heated tungsten metal coil in a light bulb, but there are other modes of generation. Incandescence is the shedding of electromagnetic radiation by a very hot material, resulting in light that can give rise to color experiences. Our dominant example of an incandescent body is the sun, where the energy is produced by what is known as nuclear fusion. The nature of incandescence as produced on Earth is most easily observed in the work of a blacksmith. An iron rod, placed in an intense coal fire, as it heats up when it reaches about 525°C will begin to give off a dull reddish glow. When viewing it in the dark, we recognize it as the source of reddish light. As the temperature of the metal increases so does the intensity of the emitted light and its energy content. Simultaneously, reddishness diminishes and the object becomes “white hot.” With further increase in temperature, it eventually assumes a bluish-white appearance. Energy is absorbed by the the iron rod from the fire and emitted in visible form by the glowing metal. The imparted energy can have many sources: thermonuclear in case of the sun; electrical in case of a light bulb; and chemical in case of burning coal. All elements can, in proper conditions, be made to show incandescence, as can many inorganic molecules. Organic molecules (those containing carbon), are usually destroyed before they show incandescence, with incandescence produced by their decomposition products (say, in case of candle wax). The nature of the emitted energy depends on the form of the incandescent material: gaseous substances and many chemical elements emit energy in one or more distinct bands; incandescent liquids and solids tend to emit energy across broad spectrum bands.
Energy absorption and incandescence are explained using the atomic model of matter. Atoms consist of protons and neutrons in the nucleus, surrounded by electrons in shells. When an atom absorbs energy, its outermost electrons get excited to higher energy levels. These excited electrons eventually fall back to lower levels, releasing energy as electromagnetic radiation. If the emitted energy has a wavelength between 400 and 700 nm, it is visible light. Significant radiation emission, or incandescence, occurs when an object's temperature reaches about 525°C.
Blackbody radiation
A blackbody is an idealized material that is a perfect absorber and emitter of energy across all wavelengths. It absorbs all incident radiation, regardless of frequency or angle of incidence, and re-emits energy perfectly based on its temperature. The theoretical emission spectrum of a blackbody can be calculated using Planck's law, which describes the intensity of radiation emitted by a blackbody as a function of wavelength for a given temperature.
Real materials often exhibit emission spectra similar to a blackbody, and this similarity allows scientists to use the concept of blackbody temperature, expressed in Kelvin, to describe the emission behavior of light sources. Even if a light source’s emission spectrum is not exactly like that of a blackbody, its color temperature can still be correlated to the temperature of a blackbody that emits light of a similar color. This is known as the correlated color temperature (CCT).
In practical terms, consider the example of a blacksmith heating an iron rod. As the rod heats up in a coal fire, it begins to glow red at around 525°C (798 K). This reddish glow intensifies and changes color as the temperature increases, moving towards white and eventually bluish-white as it reaches higher temperatures. This color change corresponds to the shifting emission spectrum of the iron, which approximates that of a blackbody at different temperatures.
Low-burning coal, with a blackbody-like spectrum at around 1800 K, emits primarily in the reddish-orange part of the visible spectrum. Incandescent light bulbs, typically operating at about 2500 K, have an emission spectrum that is also close to that of a blackbody. However, these bulbs are inefficient because most of their emitted energy is in the infrared region, which we perceive as heat rather than visible light. This inefficiency is why incandescent bulbs become very hot during operation.
In contrast, fluorescent lamps are more energy-efficient. They emit most of their energy in the visible spectrum, resulting in less wasted heat. The most energy-efficient fluorescent lamps, known as triband lamps, emit light in three specific bands around 440 nm (blue), 540 nm (green), and 610 nm (red). These wavelengths align with the peak sensitivities of the human visual system, making these lamps more efficient in terms of visible light output compared to other types of fluorescent lamps that emit across a broader spectrum.
The appearance of certain materials can change significantly depending on the spectral power distribution of the light under which they are viewed. For example, a material might look different under incandescent lighting compared to daylight. Blackbodies at temperatures of 2500 K and higher emit light that, to the human eye, appears colorless or "white" due to our adaptation to daylight. This neutral white light is perceived when objects with high reflectance functions are illuminated, making them appear white. Various light sources with different spectral power distributions can still result in light that appears white, as long as the light has a similar effect on our visual perception as daylight.
Thus, the concept of a blackbody provides a foundational understanding of how different materials emit and absorb energy. It also helps explain the color changes observed in heated objects and the efficiency differences between various types of light sources.
Luminescence
Light can be created by processes not based on the absorption of energy, known as luminescence. There are three main types of luminescence:
Electroluminescence: This occurs when electrons, influenced by an electric field, collide with particles of matter, resulting in the emission of light. Examples include sparks, arcs of light, lightning, certain lasers, and gas discharges.
Chemiluminescence: This is the production of light from chemical reactions, typically oxidations, at low temperatures. Natural chemiluminescence, also called bioluminescence, is seen in glowworms, fireflies, some deep-sea fish, decaying wood, and putrefying meat. Commercial examples include glowing liquid-filled plastic tubes.
Photoluminescence: This appears in two forms, fluorescence and phosphorescence.
- Fluorescence: Certain molecules absorb near-UV or visible light and re-emit it as visible light of a higher wavelength. Fluorescent whitening agents in detergents, for example, absorb UV radiation (300-380 nm) and emit visible light (400-480 nm), giving materials a very white appearance. Fluorescent dyes absorb and emit visible light, such as a fluorescent red dye that absorbs light from 450-550 nm and emits it at 600-700 nm. Fluorescent minerals and light tubes are also examples of fluorescence. Fluorescent light tubes have an interior coating of fluorescing phosphor compounds and contain a small amount of mercury. When excited by an electric field, mercury emits near-UV energy, which is absorbed by the phosphor compounds and re-emitted as visible light. Fluorescence stops when the energy source is interrupted.
- Phosphorescence: Some substances can store absorbed energy and continue to emit light for some time after the exciting energy is stopped. This is different from fluorescence, where light emission stops immediately once the energy source is removed, like Glow-in-the-dark materials that continue to emit light after being "charged" by exposure to light.
These luminescence processes provide diverse ways to produce light, each with unique applications and properties.
Absorption, reflection, scattering, and transmission
Light undergoes numerous transformations from creation to oblivion. When photons interact with atoms or molecules, they lose energy and are re-emitted at lower energy levels, often as infrared radiation, resulting in a loss of visible light and an increase in perceived heat. An ideal blackbody is the most efficient absorber and emitter of energy across a wide range, but real objects absorb selectively, reflecting or scattering some photons. Reflection occurs when photons bounce off a smooth surface at the same angle, while scattering happens when photons hit rough surfaces or fine particles, spreading light in many directions.
Scattering is common in natural phenomena and materials like textile fibers, water droplets in clouds, fog, smog, dust, milk, and bird feathers. The blue sky results from the scattering of shorter light wavelengths by small atmospheric particles, while clouds appear white due to the equal scattering of all wavelengths. Near sunset, the sky turns red as particles scatter shorter wavelengths, leaving longer wavelengths to dominate.
No material perfectly reflects or scatters light, but some, like pure barium sulfate and metallic mirrors, come close. Most colors we see result from a combination of wavelength-specific absorption and scattering, known as object colors, depicted by spectral reflectance curves.
Transmission is the passage of light through a transparent material, like water. If the material lacks absorbing substances, the transmitted light's spectral distribution remains unchanged. When absorbing materials like dyes are present, some light is absorbed, and the rest is transmitted, with the amount depending on the material and layer thickness. The Beer-Lambert-Bouguer law describes the absorption and transmission of light by dissolved substances.
Refraction
Refraction denotes the change in direction of light as it passes from one medium to another, such as from air to water or glass. This occurs because light changes speed when entering a different medium, bending according to the laws of refraction. Refraction is fundamental to phenomena like rainbows and image formation in cameras and eyes. In both, lenses control the refraction to focus light and form clear images. The degree of refraction depends on the optical densities of the media and the energy level (wavelength) of the photons, with higher energy photons bending more sharply.
A practical application of refraction is the use of glass prisms to separate light into its component wavelengths. When a beam of white light passes through a prism, it disperses into a spectrum of colors, visible when projected onto a white surface. Each color corresponds to a specific wavelength: blue (400-490 nm), green (490-570 nm), yellow (570-590 nm), orange (590-630 nm), and red (630-700 nm). This separation of colors can be reversed to recombine the light into white light.
The most dramatic natural example of refraction is a rainbow, where sunlight is refracted and dispersed by water droplets in the atmosphere. Refraction also causes the sparkling "fire" seen in cut crystals, diamonds, and other gemstones. However, refraction in lenses can lead to chromatic aberration, a problem where different wavelengths focus at different points, causing blurred or colored edges in images. Lenses must be corrected for this to achieve clear focus across all colors.
Interference
Puddles of water with bright multicolored bands near car repair shops or gas stations after rain, as well as the shimmering colors on butterfly wings or peacock feathers, are examples of iridescence. Unlike the scattering effect seen in blue jays, iridescent colors change in hue and intensity based on the viewing angle due to a phenomenon called interference.
Interference occurs when light waves split into separate parts that later recombine. Think of it as two synchronized swimmers: if they move perfectly together (in phase), their splash is bigger; if they move oppositely (out of phase), their splash cancels out. Similarly, light waves in phase amplify the light, while out-of-phase waves reduce the light. A common source of interference is a thin transparent film, such as oil on water or a soap bubble. The thickness of the film determines whether the reflected light waves are in or out of phase. When in phase, light of different wavelengths reflects at corresponding angles, producing pure, strong colors that change with the viewing angle. In thin films with varying thickness, like oil on water, multiple colors appear due to varying path lengths of the reflected light.
This phenomenon also occurs in nature. The wings of butterflies and feathers of birds like peacocks and hummingbirds have microscopic structures that create thin-film interference. These structures split and reflect light at different angles, resulting in vibrant, shimmering colors that change as the light or observer's position changes. Iridescence in these structures is often due to layers of chitin or keratin, creating multiple thin films that produce striking visual effects. Interference is also seen in everyday life. The colors in soap bubbles are due to thin-film interference. The varying thickness of the soap film creates a spectrum of colors that shift as the bubble moves. Similarly, anti-reflective coatings on glasses and camera lenses use destructive interference to reduce glare by causing specific wavelengths of light to cancel each other out.
Diffraction
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