Topic 12: Color

Color Subtraction Absorption When light strikes an object, it may be transmitted, absorbed, or reflected. A windowpane, for example, transmits almost all the light that strikes it. Since it does not change the light, the pane looks colorless, or clear. A black colored object, on the other hand, absorbs almost all the light that strikes it and therefore--since blackness is the absence of light-- looks dull and black.

A plaster wall both reflects and absorbs light. If the wall is white, it reflects almost all the light that falls on it. The yellowish color of light from an electric lamp changes most colors, but our eyes tend to see the colors unchanged. By looking at colored objects under pure colors of light or through colored filters we see a pronounced change. In very dim light colored objects appear gray. Under red light, red appears red but green and blue appear black. Under blue light, blue objects appear blue while red and green objects appear black. Under green light, green appears normal, but red and blue appear black. The color that is observed can be predicted using the rules of Color Subtraction, which will be developed in the laboratory.

17. When no light (black) shines in the eye, how do cell cones respond?

A. Only one cell cone is stimulated B. All cones are stimulated C. Two of the three cone types are stimulated D. None of the cones are stimulated

Perception of Colors The perceived color of an object depends both on the color of the light that shines on it, the colors absorbed by the dyes in the paper and certain other factors. When light reflected from an object enters a human eye, it passes through the cornea, the pupil, and the lens and lands on the retina. The interpretation of this information allows us to perceive our environment. When this information can be interpreted in more than one way it may be perceived incorrectly and visual illusions may arise. As we collect and process information it is extremely important to understand how we perceive images as well as how color will influence our perception of our environment

Benham Disks The Benham disk is a black and white pattern that when spun rapidly seems to produce color. Benham's disk was invented by a nineteenth-century toy maker who noticed colors in a black and white pattern he had mounted on a top. What is perceived is a result of the eye-brain system intercepting intermittent patterns of black and white light entering the eye. The best explanation involves the properties of the three types of cone cells found in the eye. Each cone type responds mainly to light from a different region of the spectrum. When white light shines in the eye, all three cone types respond equally. After white light stops entering the eye, all cones turn "off" but not immediately and not at exactly the same time. One type turns off a little sooner, and another turns off a little later. Thus, when you look at the spinning Benham disk, white light intermittently enters your eye. This makes the cones "turn off" and "turn on" at different times causing your eye-brain system to receive different responses from the three types of cones. This stimulation of the cones is perceived by the brain as color.

12. What color(s) will be observed when magenta and green colored filters are overlapped in white light?

Black

14. Using a cyan light shone on red will be observed as which of the following colors?

Black

13. Using a cyan light shone on blue will be observed as which of the following colors?

Blue

16. Using two colors, magenta and blue pigments what is the resulting color?

Blue

Self Evaluation 1. The process of combining two different colors of light to produce a third is called?

Color Addition

6. The process of mixing together two primary colors and creating a secondary color is called?

Color addition of light

18. Benham disk produces?

Colors from black and white

Pigments Pigments are chemicals that have the ability absorb certain colors and reflect others. Because of their widespread use to color our homes, cars, clothes, and in general our world we will investigate them as a specific form of Color Subtraction. Mixtures of Paint usually exhibit the complex behavior of subtractive mixing. A mixture of yellow and cyan watercolors produces green.

Colors produced by the subtraction of wavelengths, or filtering, often occur in nature. The reds and oranges of a sunset are caused by the filtering action of the sky. The sky scatters light of short wavelengths, such as blue. At midday, when the sun is overhead, the scattered blue light does not have to travel through very much air to reach a viewer. The sky looks blue because a great deal of blue light is reflected from it. But, at sunset the light must travel through much more air on its way to Earth. The blue is soon scattered, and only the colors of longer wavelengths--combined to appear orange and red--can be seen.

7. If the color blue is placed next to the color green on a wheel, what color will you see when it is moving fast.

Cyan

9. If a small dot of the color green is placed next to a small dot of the color blue, in the Sunday comics what color will you see?

Cyan

10.If a small dot of the color blue is placed next to a small dot of the color red, in the Sunday comics what color will you see?

Magenta

Color Addition There are three ways to produce color through color addition: 1) Addition or mixing of different colors of light 2) Movement of two or more different colors rapidly and 3) Color Spacing using small dots of different colors placed near one another

Mixing of Colored Lights Newton discovered that by mixing two differently colored rays of light he could produce other colors. When he projected light beams from different prisms onto a white background, he found that sometimes the new color looked like one of the other colors of the spectrum. Red and yellow, for example, could be mixed to look like the orange of the spectrum. He found that almost all colors can be made by three beams of differently colored light. The greatest number of different colors can be produced when a combination of one of the reds, a green, and a blue are used. For this reason red, green, and deep blue are called the primaries for additive color mixing. You will develop the Color Addition facts that will allow you to predict the color produced when the primary colors are mixed.

Afterimages Perhaps the best-known visual affect that shows how our eyes perceive color is the afterimage. When people watch a motion picture, they are actually observing a series of rapidly projected still pictures. During the very short interval between pictures, a person retains an image of the preceding picture. This image blends into that of the following picture, giving an impression of continuous motion. The retained image is called a positive afterimage.

Persistence of vision in the absence of a physical stimulus occurs if people look at a patch of one color for about 30 seconds and then look at a blank sheet of white or gray paper. They will probably see a patch of color that is the complement of the original color. This is called a negative afterimage

2. The color red, green and blue are called?

Primary colors of light

Complementary Colors When only two of the additive primaries are mixed in a certain amount, the resulting color is called the complementary color, or complement, of the third additive primary. When red and green light beams are mixed, the resulting color is yellow, the complementary color of blue. A mixture of red and blue makes a purplish color called magenta, the complement of green. And green and blue mixed together form cyan, the complement of red. In other words whatever color is missing to produce white is the complementary color.

Rapid Movement of Colors An object can be produced with exhibits two or more colors. If this object is moved rapidly the colors will appear to blend and appear to the viewer as though there is only one color on the object. This color that one observes can be predicted using the rules of Color Addition.

15. Using two colors, yellow and red pigments what is the resulting color?

Red

11. What are the color(s) that are subtracted when using a green colored filter on white light?

Red and Blue

5. The colors yellow, magenta and cyan are called?

Secondary colors of light

The Spectrum From earliest times the rainbow had delighted and puzzled observers. People invented myths to explain the beautiful arc of multicolored light that appeared after the rain. But a scientific answer to the puzzle of the rainbow did not come until the 17th century. Isaac Newton directed a small beam of sunlight into a darkened room through a prism. The beam produced a band of colors just like the rainbow, ranging from red, orange, yellow, green, blue and violet. He then passed each of these colors through other prisms and found that certain colors could not be further subdivided. But when he passed the whole band of colored light through a prism in reverse position, the colored band became sunlight again. From this he reasoned that white light is really a mixture of colored lights, and that each color is bent by a different amount when it passes through the prism. This difference in bending enables each color to stand out separately and be visible. The band of colored lights thus formed is called a spectrum. The rainbow is actually a spectrum, formed by sunlight passing through raindrops.

Separating light into its colors is accomplished by refraction (bending) of light in the prism. Each of the colors has a specific wavelength. The wavelength determines how much each color will bend. Red bends the least and violet the most. Scientists use the prism in a device called a spectroscope. The spectroscope reveals that the spectral pattern of light is different for various types of light sources. Light from the sun, from certain lamp filaments, and from molten metals produce a spectrum that has all colors in an unbroken array. Such a pattern is called a continuous spectrum. Incandescent gases give off only certain colors, in fine lines. Their spectra are called bright-line spectra. Scientists have obtained spectra corresponding to the different elements and have measured and charted every line. When they wish to learn the composition of a star, they photograph its spectrum and then check the lines against these charts for the elements

4. The wavelength of light determines?

The color the eye perceives

In the late 19th century the theory that light travels in the form of electromagnetic waves won acceptance. Waves are described by speed, wavelength, and frequency. In a given medium, such as air or a vacuum, all light waves travel at the same speed, but they differ in wavelength and frequency. Wavelength and frequency are inversely proportional to each other due to the observation that the longer the wavelength the lower the frequency. For the visible light spectrum, scientists commonly specify only the wavelength. Each color is associated with a range of wavelengths. The name green or red does not apply to just one color. A wide segment of the spectrum contains colors that are called green. These include blue-green, apple green, and chartreuse, as well as many intermediate greens. Another wide segment contains colors that are called red. Colors of nearly the same wavelength look exactly alike to the human eye

The colors of the spectrum range, in order, from violet, through blue, green, yellow, and orange, to red. The wavelengths of violet are the shortest, ranging from 380 to about 450 nanometers. (A nanometer is one billionth of a meter long.) Wavelengths of red are the longest, ranging from about 630 to 760 nanometers. Wavelengths shorter than those of violet are called ultraviolet radiation; wavelengths longer than those of red are infrared radiation. "Black" is the absence of color. Colors can be observed in nature as specifics wavelengths of light or can be produced by three fundamental processes that can allow us to see most of the colors of the spectrum. These three processes are Color Addition, Color Subtraction and Color Perception.

How We See Color The function of our eyes is to bring information from the outside world into our brain. If you look around a room you see things: furniture, pictures, tables, objects, and shadows. Sometimes it is hard to realize that the things you see are really pictures inside your head; parts of images formed by the brain and eye.

To form these pictures, your eyes receive light that reflects from the objects around you. Your brain and eye work together to process the information carried by this light to create an internal image of what is outside. This internal image of the outside world is not complete and your brain fills in this incomplete internal picture. We are hardly ever aware of the idiosyncrasies of our own vision.

3. The colors produced by a diffraction grating in order of increasing wavelength, is:

Violet, Indigo, Blue, Green, Yellow, Orange, Red

Filters Sometimes a substance absorbs some but not all the colors that reach it. For example, a red tomato absorbs all wavelengths but those of red, which, after bouncing from molecule to molecule within the top layers of the tomato, are redirected outward. When blue light (which does not contain red wavelengths) shines on a tomato, the blue wavelengths are absorbed and the tomato looks black because no red light is reflected. Transparent red objects such as red cellophane, red plastic, or red glass absorb all wavelengths but red ones, which they partly transmit and partly reflect. Such transparent objects are called color filters because when white light strikes them they filter out all colors except their own, which can pass through them easily. Color filters are the basis of subtractive color mixing, just as colored beams of light are the basis of additive mixing. Subtractive color mixing is a complicated procedure because the different dye molecules in two different filters may produce the same color sensation yet absorb different wavelengths of light. The description of subtractive color mixing that follows assumes that ideal filters are used.

When a beam of white light strikes a yellow filter, the wavelengths that make up yellow can pass through the filter while all other wavelengths are absorbed. Since yellow is a mixture of green and red light, the wavelengths of those colors pass through, but the wavelengths of blue--the complement of yellow--are absorbed. A yellow filter is sometimes called minus-blue, since it can filter out blue light. Similarly, a magenta filter allows wavelengths of red and blue to pass but absorbs wavelengths of its complement, green. For this reason, magenta is sometimes called minus-green. If a yellow filter (minus-blue) is placed on top of a magenta filter (minus-green) and a beam of white light is passed through them, the yellow filter absorbs blue, the magenta filter absorbs green, and only red light emerges. A cyan filter (minus-red) absorbs its complement, red. If a yellow, a cyan, and a magenta filter are aligned in front of a beam of white light, all three of the additive primaries are absorbed and no light emerges. This is called subtractive color mixing because the filters absorb, or subtract, color from a beam of light. The color that is observed using one or more filters can be predicted using the rules of Color Subtraction

8. If the color green is placed next to the color red on a wheel, what color will you see when it is moving fast?

Yellow