ch28_cdhw

Chapter 28: C o l o r  **Definitions:** 28.1: **specturm**:for sunlight and other white light, the spread of color seen when light is passed through a prism or a diffraction grading. In general, the spread of radiation by frequency, so that each frequency appears at a different position. 28.1: **white light**: Light, such as sunlight, that is a combination of all the colors. Under white light, objects appear white and colored objects appear in their individual colors. 28.3: **Pigment**: A material that selectively absorbs colored light. 28.5: **additive primary colors**: Red, blue, and green light. These colors, when added together, produce white light. 28.6: **complementary colors**: Two colors of light beams that, when added, together appear white. 28.7: **subtractive primary colors**: The colors of magenta, yellow, and cyan. These are the three colors most useful in color mixing by subtraction. 28.11: **spectroscope**: An instrument used to seperate the light from a hot gas or other light source into its constituent frequencies. 28.11: **line spectrum**: Pattern of distinct lines of color, corresponding to particular wavelengths, that are seen in a spectroscope when a hot gas is viewed. All definitions drawn from //Conceptual Physics: Third Edition.//

 **28.1: The Color Spectrum** Isacc Newton showed that sunlight is composed from all the colors of the rainbow by passing it through a clear glass prism. He called this the color **spectrum** and noted they are formed in order from red to violet. Sunlight is an example of **white light**. Newton showed that colors in the spectrum do not result because of the prism but because of light itself. He proved it by putting a second prism in front of the first causing the colors to recombine in to white light. Black is the absence of light. Objects look black when they absord all visible light frequencies. They do not absord all the frequencies, however, because if they did you wouldn't be able to see them.

Colors seen result from the way objects reflect light. Light is reflected simiary to the was sound is reflected from a tuning form when it is set into a vibration by another. Different materials absord and emit different radiations because of their natural frequency. In an object where the amplitude of the oscilliation is large light is absorbed. However, at frequencies below and above resonate frequencies light is reemitted, not absorbed. This results in reflections. Almost all materials absorb some frequencies of light and relect the rest. The object appears the same color as the frequency of light it reflects. For example, if an object appears blue, it absorbs all frequencies of light and reflects blue. The reflected colors from most objects are not single frequencies, but mulitple frequencies. Something that appears yellow could be a mixture of red and green frequencies because they combine to create yellow. An object can only reflect light of frequencies present in the illuminating light. Certain lamps emit different frequencies causing objects in the lamp light to appear different colors. For example, candle light emits yellow light, causing the objects in the candle light to look yellowish. The color an object looks is dependent on the light source.
 * 28.2: Color by Reflection**

A transparent objects color depends on the color it transmits. If a piece of glass were green it appears green because it mostly transmits green light. **Pigment** is the material in the glass that selectively absorbs colored light. Electrons in pigment atoms only absord certain frequencies of light. This is transmitted from atom to atom in the glass, causing it only to appear mostly one color. The energy created from the absorbtion of the frequencies causes the glass to get warm because of the use of kinetic energy.
 * 28.3: Color by Transmission**

Light from the sun is considered white light, and is composed of all visible frequencies. The lowest frequencies in the sunlight are red with the highest being yellow-green light. Because of this humans are most sensitive to yellow-green light. The distribution of brightness versus frequency is called the radiation curve of sunlight.
 * 28.4: Sunlight**

When all visble frequencies are mixed together it produces white light. White can be created from the combination of only red, green, and blue light also. When the frequencies of white light are divided into three regions, the lowest frequency is red, the middle is green, and the highest frequency is blue. When the red and green frequencies combine, it appears cyan. The low and middle frequencies combine to appear yellow, and the low and high frequencies combine to appear magenta. Red, blue, and green are called the **additive primary colors** because by combining the three and adjusting the brightness you can create almost any color.
 * 28.5: Mixing Colored Light**

Red + Green = Yellow Red + Blue = Magenta Blue + Green = Cyan Yellow + Blue = White Magenta + Green = White Red + Cyan = White When two colors are added or combined to get white, they are called **complementary colors**. Every color has a complementary color. You can also reverse it and start with white light and subtract a color from it, giving you the complementary color.
 * 28.6: Complementary Colors **

The mixing of paint is completely different than mixing colored light. Paints contain particles of pigment that give them their colors by absorbing and reflecting certain frequencies. The pigments in paint absorb a wide range of frequencies and also reflect a wide range. This is called color mixing by subtraction, which differs from mixing colored light, known as color mixing by addition. Magenta, yellow, and cyan are the **subtractive primary colors**, which are used in printing.
 * 28.7: Mixing Colored Pigments**

Light is scattered from molecules and specks of matter in the atmosphere. The light enters into nitrogen and oxygen molecules in the atomosphere and is reemitted and sent in all directions. The ultraviolet light that passes through the atmosphere is scattered by particles. Violet light is scattered the most, then blue. Human eyes are more sensitve to blue which makes the sky appear blue. In an area where there are more particles in the atmosphere, the sky appears less blue. The sky becomes more blue when the particles have been washed away. Higher in the atmosphere there are less molecules to scatter the light, making it appear darker. When there are no molecules, it appears black.
 * 28.8:Why the Sky is Blue**

Nitrogen and oxygen molecules scatters the lowest frequencies of light the least, causing red, orange, and yellow light to be transmitted through the atmosphere more readily than violet and blue. Red light is scattered the least, and passes through more atmosphere without encountering particles and molecules. When the atmoshpere is thick, the lower frequency light is transmitted, while higher frequency light scatters. During a sunset, the sunlight has to travel by a longer path through the atomsphere than during the day. As the sun lowers, the path through the atmosphere increases. The colors of the sun and sky follow the rules for color mixing. When blue is subtracted from white, yellow is left. When violet is subtracted, orange is left, and when green is subtracted, magenta is left. However, the amount of scattering all depends on the conditions of the sky.
 * 28.9: Why Sunsets are Red**

The ocean reflects the color of the sky. The color of water by itself is a pale greenish-blue. Nearly all visible frequencies of light are transparent to water. Water molecules absorb infared light because the resonate at infared frequencies. The energy of the infared waves turns to kinetic energy, causing the water to warm. Water molecules absorb red light. Since the complementary color of red is cyan, that is the color reflected by the water.
 * 28.10 Why Water is Greenish Blue**

When made to emit light, every element has its own color. If the atoms are so far apart that their vibrations are not interrupted, their true colors are emitted. This occurs when atoms are made to glow in the gaseous state. For example, neon gas glows red, mercury vapor is bluish-violet, and helium glow pink. The light from glowing elements can be analyzed with a **spectroscope**. Spectroscopes display the spectra of light from gases and light sources. The spectra is viewed through a magnifying eye piece on the spectroscope. When analyzed through a spectroscope, the colors of glowing elements are the combination of a variety of different frequencies of light. The spectrum of an element is not a band of color, but a series of lines. These lines correspond to a distinct frequency of light, called the **line spectrum**. Each element's light makes its own pattern of lines because each element has a different combination of electrons that emit distinct frequencies of light.
 * 28.11: The Atomic Color Code -- the Atomic Spectra**