ch28_kbrl

Chapter 28: Color 28.1 The Color Spectrum

Spectrum: for sunlight and other white light, the spread of colors seen when the light is passed through a prism or diffraction grating. In general, the spread of radiation by frequency, so that each frequency appears at a different position. Newton looked at the spectrum and noticed that it formed red, orange, yellow, green, blue, and violet. White light: light such as sunlight that is a combination of all the colors. Under white light, white objects appear white and colored objects appear in their individual colors.


 * Ex: Sunlight

Newton proved that the colors that were in the spectrum were properties of a prism but not of the white light. White light is not a color but it is actually a combination of many colors. Black is similar to white it isn’t a color either. Some objects appear black and that is because all colors are absorbed and they are absorbed at the same frequencies. Some examples of good absorbers of light is carbon soot, and black velvet. Some polished surfaces can look black but it all depends on the conditions. An example of this would highly polished razor blades. They are not black but when they are stacked together they look black. The reason they look black is because light gets between them and it gets trapped. 

28.2 Color by Reflection

White light shines on an object and some of the light is absorbed and some of the light is reflected. The reflected light is responsible for the color of the object.

If an material absorbs all the light that shines on it then it wont reflect anything and it will appear black.

When light shines on a flower some of it is absorbed by the cells and some of it is reflected. If a flower cell contains chlorphyll will absorb light with alot of frequencies and they will then reflect green. Since they reflect green it makes the flower look green. The petals on a rose reflect mostly red light with a little bit of blue. The petals on a yellow flower such as a daffodil reflect red, green and yellow. Something that is yellow is mostlikly a mixture of colors without blue and violet.



 
 * Ex: a candle flame has higher frequencies so the light that comes from a candle looks yellow.
 * Ex: an incandescent lamp has visible frequencies but there are more lower frequencies in it so this would make it more red.




 * Ex: a fluorescent lamp has higher frequencies so there is alot of blue coming from these type of light.

28.3 Color by Transmission

When you have a transparent object the color of the object depends on the color of the light that is being transmitted through it. A piece of glass that is red looks red because it transmits all the colors that make up white light. A piece of glass that is blue looks blue because it transmits mostly blue and it absorbs the other colors.



Pigment: a material that selectively absorbs colored light.

28.4 Sunlight In a sun white light is a mixture of all the visible frequencies.



This graph is called a radiation curve. The curve shows that yellow-green is the brightest part of the sun. This is the reason why humans are most sensitive to yellow-green light. More and more fire trucks are being painted yellow-green for this very reason. It is also easier to see yellow-green at night too.

28.5 Mixing Colored Light A combination of red, green, and blue light of equal brightness overlap they appear white on a screen. If you combine red and green light it appears to be yellow. When you overlap red and blue light they make a color called magenta. Overlapping green and blue light produce a color called cyan.

A way to understand this is to break the colors into frequency of white light. The low frequency would be red, the middle frequency is green and the high frequency is blue. If you combine the low and middle frequency the color appears to be yellow. When you combine the middle and the high frequency you get cyan (greenish blue). When you combine the low and high frequency you get magenta (bluish red).

All colors can be made by simply overlapping light and then fixing the brightness of the light.

Additive primary colors: red, blue and green light. These colors when added together produce white light.

28.6 Complementary Colors

red + green = yellow yellow + blue = white red + blue = magenta magenta + green = white blue + green = cyan cyan + red = white

If you were to add in a third color your result would be white.

Complementary Colors : two colors of light beam that when added together appear white.

If you were to take white light and subtract the color you would appear to be a complement of the one you subtracted.
 * Ex: yellow and blue are complementary becuase yellow is red and green combined. Red, green and blue make white.

28.7 Mixing Colored Pigments Mixing paints is completely different then mixing colored light. Pigments have small particles in them and they produce the color by absorbing light of a specific frequency and then they reflect light of another frequency. Pigments reflect a mixture of colors.

Blue paint reflects blue light but it can also reflect violet and green. It can absorb red, orange and yellow light.

Yellow paint reflects yellow light but it can reflect red, orange and green. It absorbs blue and violet light.

Mixing these two paints will be able to absorb every color except green. Mixing these two colors will only reflect green. This is called color mixing by subtraction.

When you cast light on a stage you need to use the rules of color addition so that many colors are produced. When you mix paint you use the rules of color subtraction. Subtractive primary colors: the colors of magenta, yellow and cyan. These are the three colors most useful in color mixing by subtraction.

Color Subtraction:



28.8 Why the Sky is Blue

<span style="color: rgb(255,0,0); font-family: Georgia, serif;">Scatters: <span style="font-family: Georgia, serif;">to absorb sound or light and reemit it in all directions When light passes through particles in the atmosphere some of the light is absorbed and scattered by other particles that go through it. The short wavelengths (violet and b <span style="font-size: 10pt; font-family: Georgia, serif;">lue) get scatted which cause the sky to appear blue. The rest of the color light will shine through the clouds which causes the sun to appear yellow.



28.9 Why Sunsets Are Red

When the sun is in the sky and it is lower the sunlight has a long path to reach your eyes. When this happens the light is more likely to scatter and this leaves only red and orange light to reach the earth.



28.10 Why Water is Greenish Blue

When you look at water you will notice that it absorbs a small amount of red light. When you look at a bottle of water you most likely won’t see the coloration but when you see a large body of water the red that isn’t present makes the water looks greenish blue.



28.11 The Atomic color Code Atomic Spectra Spectroscope: an instrument used to separate the light from a hot gas or other light source into its constituent frequencies Line Spectrum: pattern of distinct lines of color, corresponding to particular wavelengths that are seen in a spectroscope when a hot gas is viewed.



W o r k s C i t e d : <span style="font-family: Georgia, serif;"> <span style="font-family: Arial; mso-bidi-font-size: 10.0pt; mso-fareast-font-family: Arial; msobidifontsize: 10.0pt; msofareastfontfamily: Arial; msolist: Ignore;">• <span style="font-size: 10pt; font-family: Georgia; mso-bidi-font-size: 13.0pt; mso-bidi-font-family: Georgia; mso-bidi-language: EN-US;">Conceptual Physics: Third Edition with Expanded Technology by Paul G. Hewitt <span style="font-family: Arial; mso-bidi-font-size: 10.0pt; mso-fareast-font-family: Arial; msobidifontsize: 10.0pt; msofareastfontfamily: Arial; msolist: Ignore;">• <span style="font-size: 10pt; font-family: Georgia; mso-bidi-font-size: 13.0pt; mso-bidi-font-family: Georgia; mso-bidi-language: EN-US;">[] <span style="font-family: Arial; mso-bidi-font-size: 10.0pt; mso-fareast-font-family: Arial; msobidifontsize: 10.0pt; msofareastfontfamily: Arial; msolist: Ignore;">• <span style="font-size: 10pt; font-family: Georgia; mso-bidi-font-size: 13.0pt; mso-bidi-font-family: Georgia; mso-bidi-language: EN-US;">[] <span style="font-family: Arial; mso-bidi-font-size: 10.0pt; mso-fareast-font-family: Arial; msobidifontsize: 10.0pt; msofareastfontfamily: Arial; msolist: Ignore;">• <span style="font-size: 10pt; font-family: Georgia; mso-bidi-font-size: 13.0pt; mso-bidi-font-family: Georgia; mso-bidi-language: EN-US;">[] <span style="font-family: Arial; mso-bidi-font-size: 10.0pt; mso-fareast-font-family: Arial; msobidifontsize: 10.0pt; msofareastfontfamily: Arial; msolist: Ignore;">• <span style="font-size: 10pt; font-family: Georgia; mso-bidi-font-size: 13.0pt; mso-bidi-font-family: Georgia; mso-bidi-language: EN-US;">[] <span style="font-family: Arial; mso-bidi-font-size: 10.0pt; mso-fareast-font-family: Arial; msobidifontsize: 10.0pt; msofareastfontfamily: Arial; msolist: Ignore;">• []