Visible spectrum

The visible spectrum is a range of colors contained within light. These colors are red, orange, yellow, green, blue, indigo, and violet. Humans perceive these colors due to subtle differences in the wavelengths of the visible light energy and the different ways that objects absorb or reflect light. People use prisms, both natural and artificial, to study the colors and their various characteristics. Breakthroughs by scientists like Sir Isaac Newton have added greatly to human understanding of the visible spectrum.

Visible Light

The visible spectrum is contained within light. Light is a visible form of electromagnetic radiation, a form of energy that travels through space at an extremely high speed. This energy takes on a wavelike structure with low points (troughs) and high points (crests). The distance between crests is called a wavelength. The size of a wavelength determines what form the energy takes. Humans have discovered special uses for each form of electromagnetic energy, but light is likely the most important of all. Light gives humans their ability to see.

Colors of the Visible Spectrum

When light waves travel together as a single unit, they appear to be white. This is why sunlight and most other natural lights appear whitish. Taking away all light will yield the opposite color—black. Closer inspection will show, however, that white light is actually composed of a spectrum, or wide range, of distinct and vivid colors. This range of colors is known as the visible spectrum. Because of the visible spectrum, humans have the ability to perceive the colors of almost everything that is exposed to light, from ants and people to mountains and other planets.

Just as waves of electromagnetic radiation form wavelengths, so too do the waves within the visible spectrum. The visible wavelengths range from approximately 400 to 700 nanometers (abbreviated as nm). Within this extremely tiny span are special wavelengths that correspond to each separate color. The longest visible wavelength, around 650 nm, creates red. The next wavelength, around 590 nm, produces orange. Yellow (around 570 nm), green (around 510 nm), blue (around 475 nm), and indigo (around 445 nm) follow. Finally, the shortest wavelength of the visible spectrum, occurring around 400 nm, produces violet.

Scientists and students have long remembered the colors red, orange, yellow, green, blue, indigo, and violet in order from longest to shortest wavelength by using the mnemonic device "ROY G. BIV." In modern times, though, many scientists have questioned whether indigo should be considered a distinct color, and they sometimes exclude it from the list.

The colors of the visible spectrum do not represent all the colors people can see. Other familiar colors can be derived by mixing spectral colors together or by mixing them with black or white. For example, the color pink, while not represented in the spectrum, is created by mixing red and white.

The actual perception of color occurs because objects absorb and reflect light in different ways. The wavelengths that are reflected most strongly become the perceived colors of an object. For instance, the leaves of a tree absorb all colors except green, which is reflected. The reflected green color meets the eyes of the observer and causes him or her to perceive the leaves as being green. In a similar manner, a red hat reflects red, a yellow bus reflects yellow, and so on. Some objects, like mirrors, are highly reflective and absorb very little light while reflecting all the rest. Other objects, like glass windows, simply allow light to pass through. These objects are considered transparent.

Study of the Visible Spectrum

Humans have always been aware of the vibrant colors that surrounded them. However, the exact nature of these colors and the visible spectrum remained a mystery for thousands of years. Scientists began to learn more about how colors form when they began using prisms, tools that break apart light. One well-known example of a prism is a glass tool cut to look like a crystal. When light strikes this tool, the different wavelengths of light are broken and bent to travel in different directions. Because of this effect, each color in the light becomes visible. Prisms may also occur naturally, most often in the form of rainbows. In a rainbow, small water droplets hanging in the air serve as tiny prisms, splitting sunlight into its component colors. The bars of a rainbow display the full spectrum of colors, from red to violet.

Prisms fascinated but confused early scientists, most of whom believed that the prisms were somehow adding color to white light. A breakthrough in the study of the visible spectrum came in 1665, when scientist Sir Isaac Newton created a simple but ingenious experiment. Newton held up a prism to a beam of sunlight and noted that the prism created a band of diverse colors. This was the expected result, but Newton took it a step further. He held up a second prism upside down behind the first. The colored light entered the second prism and reformed into a unified beam of white light. Newton had proven that prisms do not add color to light; rather, they break open light to expose the colors hidden within.

Many subsequent scientists explored Newton's results and staged new experiments, greatly advancing the study of the visible spectrum. One of the next important discoveries was that colors can be distinguished by special properties such as hue, saturation, and brightness. Hue is a term for the color itself, derived from its place on the spectrum of visible light. Saturation refers to whether the color is mixed with white. A color that is fully saturated has no traces of white in it. Brightness is concerned with whether the color gives off illumination and how strongly it reflects off objects.

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