Most of the radiation that surrounds us cannot be seen, but can be detected by dedicated sensing instruments. The visible spectrum constitutes but a small part of the total radiation spectrum. A common feature of all these colored compounds, displayed below, is a system of extensively conjugated pi-electrons. The deep orange hydrocarbon carotene is widely distributed in plants, but is not sufficiently stable to be used as permanent pigment, other than for food coloring. A rare dibromo-indigo derivative, punicin, was used to color the robes of the royal and wealthy. These included the crimson pigment, kermesic acid, the blue dye, indigo, and the yellow saffron pigment, crocetin. Many of these were inorganic minerals, but several important organic dyes were also known.

Green is unique in that it can be created by absoption close to 400 nm as well as absorption near 800 nm.Įarly humans valued colored pigments, and used them for decorative purposes. Thus, absorption of 420-430 nm light renders a substance yellow, and absorption of 500-520 nm light makes it red. Here, complementary colors are diametrically opposite each other. This relationship is demonstrated by the color wheel shown on the right. The remaining light will then assume the complementary color to the wavelength(s) absorbed. When white light passes through or is reflected by a colored substance, a characteristic portion of the mixed wavelengths is absorbed. In horizontal diagrams, such as the one on the bottom left, wavelength will increase on moving from left to right. The wavelengths of what we perceive as particular colors in the visible portion of the spectrum are displayed and listed below. Other common colors of the spectrum, in order of decreasing wavelength, may be remembered by the mnemonic: ROY G BIV. The longest visible wavelength is red and the shortest is violet. Visible wavelengths cover a range from approximately 400 to 800 nm. Frequency is the number of wave cycles that travel past a fixed point per unit of time, and is usually given in cycles per second, or hertz (Hz). Wavelength is defined on the left below, as the distance between adjacent peaks (or troughs), and may be designated in meters, centimeters or nanometers (10 -9 meters). Electromagnetic radiation such as visible light is commonly treated as a wave phenomenon, characterized by a wavelength or frequency. As shown on the right, the component colors of the visible portion can be separated by passing sunlight through a prism, which acts to bend the light in differing degrees according to wavelength. Although we see sunlight (or white light) as uniform or homogeneous in color, it is actually composed of a broad range of radiation wavelengths in the ultraviolet (UV), visible and infrared (IR) portions of the spectrum. In this respect the human eye is functioning as a spectrometer analyzing the light reflected from the surface of a solid or passing through a liquid. Thus, quinone is yellow chlorophyll is green the 2,4-dinitrophenylhydrazone derivatives of aldehydes and ketones range in color from bright yellow to deep red, depending on double bond conjugation and aspirin is colorless. UV-Visible Spectroscopy Visible and Ultraviolet SpectroscopyĪn obvious difference between certain compounds is their color.

Spectra 1 vs 2