Chapter 27 Color

What is the color of the peak frequency of solar radiation?

The peak frequency of sunlight is yellow-green.

What part of the visible electromagnetic spectrum is most absorbed by water?

The red part of the electromagnetic spectrum is most absorbed by water

Why will the leaves of a red rose be warmed more than the petals when illuminated with red light? Discuss how this relates to people in the hot desert wearing white clothes.

The red petals of a red rose reflect red light, while the green leaves absorb red light. The energy absorbed by the leaves tends to increase their temperature. White material reflects radiation and is therefore worn by those who do not wish to be warmed by absorbing radiant energy.

When white light is shone on red ink dried on a glass plate, the color that is transmitted is red. But the color that is reflected is not red. What is it?

The reflected color is white minus red, or cyan.

Why are red, green, and blue called the additive primary colors?

These three colors of equal brightness add together to produce white.

Volcanic emission spew fine ashes in the air that scatter red light. What color does a full Moon appear to be through these ashes?

Through the volcanic emissions, the Moon appears cyan, the complementary color of red.

Why does the blood of injured deep-sea divers look greenish-black in underwater photographs taken with natural light, but red when flash is used?

Deep in water red is no longer present in light, so blood looks black. But there is plenty of red in a camera flash, so the blood looks red when so illuminated.

What happens to light when it falls on a material that has a natural frequency above or below the frequency of the light?

It is reemitted.

Red + blue =

Magenta.

Which interact more with high-pitched sounds: small bells or large bells?

Small bells interact more with high-frequency sounds.

Which interact more with high-frequency light: small particles or large particles?

Small particles interact more with higher frequency light

Tennis balls used to be white. What is their color today. and why?

Tennis balls are yellow-green to be more visible; they match the color to which we are most sensitive.

White - blue =

Yellow

Of these sources of blue light (a) TV screen, (b) the sky, and (c) lakes of the Canadian Rockies, which are due to scattering?

(b) and (c)

Your friend reasons that magenta and yellow paint mixed together will produce red because magenta is a combination of red and blue and yellow is a combination of red and green---and that the color in common is red. Do you agree or disagree, and why?

Agree, because the "light mathematics" is correct.

Your friend says that the reason the distant dark mountains appear blue is because you're looking at the sky between you and the mountains. Do you agree or disagree?

Agree.

Complementary colors

Any two colors of equal brightness that, when added, produce the sensation of white light.

Why is the sky a darker blue when you are at high altitudes? (Hint: What color is the "sky" on the Moon?)

At higher altitudes, there are fewer molecules above you and therefore less scattering of sunlight. This results in a darker sky. The extreme, no molecules at all, results in a black sky, as on the Moon.

What occurs when the outer electrons that buzz about the atomic nucleus encounter electromagnetic waves?

When electromagnetic waves encounter the electrons around an atom, the vibrations of the electromagnetic wave force the electrons to vibrate as well

What happens to light when it falls on a material that has a natural frequency equal to the frequency of the light?

When light falls on a material with a matching natural frequency, it is absorbed.

What color results when red is subtracted from white light?

When red is subtracted, the result is cyan.

When something is painted red, what color is most absorbed?

When something is painted red, it reflects red light and absorbs cyan light

Why does water appear cyan?

When water absorbs the red part of the electromagnetic spectrum, the color reflected is cyan which makes the water appear cyan.

The radiation curve of the Sun (see Figures 27.7 and 27.8) shows that the brightest light from the Sun is yellow-green. Why, then, don't we see the Sun as yellow-green?

We see not only yellow-green but also red and blue. All together, they mix to produce the white light we see. And, due to atmospheric scattering, the Sun is yellowish.

Is it scattering or reflection that accounts for the whiteness of a cloud?

A cloud is white because it reflects all the colors of sunlight equally.

Which warms more quickly in sunlight: a colorless or a colored piece of glass? Why?

A colored piece of glass absorbs light and warms more quickly.

Why does a white piece of paper appear white in white light, red in red light, blue in blue light, and so on for every color?

A piece of paper that appears white in sunlight has the property of reflecting any color that is incident on it.

What is a radiation curve?

A radiation curve is a plot of brightness vs. frequency for light.

Streetlights that use high-pressure sodium vapor produce light that is mainly yellow with some red. Why aren't dark blue police cars advisable in a community that uses these streetlights?

Orange-yellow is complementary to blue, which combine to black. Cars would be difficult to see under such light.

To what color of light are our eyes most sensitive?

Our eyes are most sensitive to yellow-green light

What color of light will be transmitted through overlapping cyan and magenta filters?

Purple will be seen. See Figure 27.12

Why are red and cyan called complementary colors?

Red and cyan add to produce white.

What is the resulting color of equal intensities of red light and cyan light combined?

Red and cyan light combined form white light

On a TV screen, red, green, and blue spots of fluorescent materials are illuminated at a variety of relative intensities to produce a full spectrum of colors. What dots are activated to produce yellow? Magenta? White?

Red and green produce yellow; red and blue produce magenta; red, blue and green produce white.

What color of visible light has the longest wavelength? The shortest wavelength?

Red has the longest wavelength; violet has the shortest wavelength.

What color light is transmitted through a piece of red glass?

Red light is transmitted through red glass.

What frequency ranges of the radiation curve do red, green, and blue light occupy?

Red occupies the low frequency end of the visible spectrum. Green occupies the middle frequency part of the visible spectrum. Blue occupies the high frequency end of the visible spectrum.

From Figure 27.9, find the complements of cyan, of yellow, and of red.

Red, blue, cyan.

When white light passes through a prism, light of which color bends the least upon emerging?

Red.

Why does the Sun look reddish at sunrise and sunset but not at noon?

Scattering of high-frequency blue light occurs all along the path of sunlight, so the long path at sunrise or sunset finds much blue missing. What remains is light of lower frequencies, which accounts for the reddish color of the Sun at these times. At noon the path through the atmosphere is shorter and less scattering occurs.

Pilots sometimes wear glasses that transmit yellow light and absorb light of most other colors. Why does this help them see more clearly?

Such glasses eliminate the distraction provided by the more strongly scattered blue and violet light yet let the pilot see in a frequency range where the eye is sensitive. (Glasses that transmit predominantly red would also get rid of the scattered blue and violet light but would provide light to which the eye is not very sensitive.)

What is the effect on the color of a cloud when it contains an abundance of large droplets?

When a cloud has an abundance of large droplets, the large drops absorb more light and make the cloud look dark.

Distant snow-covered mountains reflect a lot of light and are bright. Very distant ones look yellowish. Why? (Hint: What happens to the reflected white light as it travels from the mountains to us?)

Bright snow-covered mountains appear yellow because the blue in the white light they reflect is scattered on its way to us. By the time the light reaches us, it is weak in the high frequencies and strong in the low frequencies---hence, it is yellowish. Snow-covered mountains much farther away have an orange tinge for the same reason a sunset appears orange.

White - red =

Cyan

If molecules in the sky scattered low-frequency light more than high-frequency light, what color would the sky be? What color would sunsets be?

If low-frequency light were scattered, the noontime sky would appear reddish-orange. At sunset, more reds would be scattered by the longer path of the sunlight, and the sunlight would be predominantly blue and violet. So sunsets would appear blue!

Can stars be seen from the Moon's surface in the "day-time" when the Sun is shining?

From the surface of the Moon, both the Sun and the stars are clearly visible. This is because there is no skylight (scattered sunlight) to overwhelm the starlight.

Check Figure 27.9 to see whether the first three statements are accurate. Then complete the last statement. (All colors are combined by the addition of light.) Red + green + blue = white Red + green = yellow = white - blue Red + blue = magenta = white - green Green + blue = cyan = white - _____________

Green + blue = cyan = white - red

How would the whiteness of snow appear if Earth's atmosphere were several times denser?

If the atmosphere were several times denser, the sunlight reaching the Earth would be predominantly low frequencies because most of the blue light would be scattered away. Snow would likely appear orange at noon, and a deep red when the Sun is not directly overhead.

How could you use the spotlights at a play to change the performers' clothes suddenly from yellow to black?

If the yellow clothes of stage performers are illuminated with a complementary blue light, they will appear black.

Distant dark mountains are bluish. What is the source of this blueness? (Hint: What is between us and the mountains we see?)

If we look at distant dark mountains, very little light from them reaches us, and the blueness of the atmosphere between us and them predominates. The blueness we attribute to the mountains is actually the blueness of the low-altitude "sky" between us and the mountains!

What part of the electromagnetic spectrum is most absorbed by water?

Infrared light is most absorbed by water.

Red sunrises occur for the same reason as red sunsets. But sunsets are usually more colorful than sunrises---especially near cities. What is your explanation?

Sunsets follow the activities of humans and other life that put dust and other particles in the air. So the composition of the sky is more varied at sunset.

If you look with a magnifying glass at pictures in a book or magazine that are printed in full color, you'll notice three colors of ink plus black. What are these colors?

The 3 other colors seen besides black in full color prints are cyan, magenta and yellow

Why does the color of sunsets vary from day to day?

The color of sunsets varies from day to day based on the number of larger particles in the atmosphere.

In a boutique store with only fluorescent lighting, a customer insists on taking dresses into the daylight at the doorway to check their color. Discuss whether she is being reasonable.

The customer is being reasonable in requesting to see the colors in the daylight. Under fluorescent lighting, with its predominant higher frequencies, the bluer colors rather than the redder colors are accented. Colors appear quite different in sunlight.

Why is the foam of root beer white, while the beverage is dark brown?

The foam is composed of tiny bits of liquid that scatter light as a cloud does.

Why are the interiors of optical instruments intentionally black?

The interior coating absorbs rather than reflects light and therefore appears black. A black interior in an optical instrument will absorb any stray light rather than reflecting it and passing it around the interior of the instrument to interfere with the optical image.

When red light shines on a red rose, why do the leaves become warmer than the petals?

The leaves absorb the energy of red light, rather than reflect it, ans so they become warmer.

When green light shines on a rose, why do the petals look black?

The petals absorb rather than reflect the green light. Because green is the only color illuminating the rose and because green contains no red to be reflected, the rose reflects no color and appears black.

If you hold any small source of white light between you and a piece of red glass, you'll see two reflections from the glass: one from the front surface and one from the back surface. What color is each reflection?

The reflection from the front surface is white because the light doesn't go far enough into the colored glass to allow the absorption of nonred light. Only red light reaches the back surface because the pigments in the glass absorb all the other colors, and so the back reflection is red.

Why does the sky normally appear blue?

The sky normally appears blue because the blue end of the spectrum is scattered most by sunlight.

Why does the sky sometimes appear whitish?

The sky sometime appears whitish because there is a greater amount of larger particles in the atmosphere which scatter the other lower frequencies as well, forming white light

Comment on this statement: "Oh, that beautiful red sunset is just the leftover colors that weren't scattered on their way through the atmosphere".

The statement is true. A more positive tone would omit the word just because the sunset is not just the leftover colors; it is those colors that weren't scattered in other directions.

Subtractive primary colors

The three colors of absorbing pigments---magenta, yellow, and cyan---that, when mixed in certain proportions, can reflect any other color in the visible-light part of the electromagnetic spectrum.

Additive primary colors

The three colors---red, blue, and green---that, when added in certain proportions, produce any other color in the visible-light part of the electromagnetic spectrum and can be mixed equally to produce white light.

What are the subtractive primary colors?

The three subtractive primaries are cyan, yellow, and magenta (CYM).

What is the evidence for the statement that white light is a composite of all the colors of the spectrum?

White light is a composite of all colors of the spectrum and can be shown by passing sunlight through a prism and being able to see the rainbow colored spectrum of visible light.

For the projection of light, the primary colors are RGB. For light we see by reflection from opaque surfaces, the primary colors are CMY. Is this correct?

Yes! And for reflected light, toss in black with the dyes to get primo images.

Since white light is all the colors added together, is black simply the absence of light?

Yes.

Stare intently at an American flag. Then turn your view to a white area on a wall. What colors do you see in the image of the flag that appears on the wall?

You see the complementary colors due to retina fatigue. The blue will appear yellow, the red cyan, and the white black. Try it and see!