the light we see

the colors we see

The human eye can distinguish only red, green, and blue light. These three colors of light are called primary colors. All other colors of light can be created by combining the primary colors. As you can see in Figure 22.7, when red and green light combine, they form yellow. When red and blue light combine, they form magenta, a dark pinkish color, and when blue and green light combine, they form cyan, a bluish green color. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram in Figure 22.7. When all three primary colors combine, they form white light. White is the color of the full spectrum of visible light when all of its wavelengths are combined. You can explore the colors of visible light and how they combine with the interactive animations at this URL: .

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pigments

Many objects have color because they contain pigments. A pigment is a substance that colors materials by reflecting light of certain wavelengths and absorbing light of other wavelengths. A very common pigment is chlorophyll, which is found in plants. This dark green pigment absorbs all but green wavelengths of visible light. It is responsible for capturing the light energy needed for photosynthesis. Pigments are also found in paints, inks, and dyes. Just three pigments, called primary pigments, can be combined to produce all other colors. The primary pigment colors are the same as the secondary colors of light: cyan, magenta, and yellow. The printer ink cartridges in Figure 22.8 come in just these three colors. They are the only colors needed for full-color printing.

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separating colors of light

A prism, like the one in Figure 22.5, can be used to separate visible light into its different colors. A prism is a pyramid-shaped object made of transparent matter, usually clear glass. It transmits light but slows it down. When light passes from the air to the glass of the prism, the change in speed causes the light to bend. Different wavelengths of light bend at different angles. This causes the beam of light to separate into light of different wavelengths. What we see is a rainbow of colors. Look back at the rainbow that opened this chapter. Do you see all the different colors of light, from red at the top to violet at the bottom? Individual raindrops act as tiny prisms. They separate sunlight into its different wavelengths and create a rainbow. For an animated version of Figure 22.5, go to the URL: http://en.wikipedia.org/wiki/File:Light_dispersion_conce

colors of objects

We see an opaque object, such as the apple in Figure 22.6, because it reflects some wavelengths of visible light. The wavelengths that are reflected determine the color that the object appears. For example, the apple in the figure appears red because it reflects red light and absorbs light of other wavelengths. We see a transparent or translucent object, such as the bottle in Figure 22.6, because it transmits light. The wavelength of the transmitted light determines the color that the object appears. For example, the bottle in the figure appears blue because it transmits blue light. The color of light that strikes an object may also affect the color that the object appears. For example, if only blue light strikes a red apple, the blue light is absorbed and no light is reflected. When no light reflects from an object, it looks black. Black isnt a color. It is the absence of light.

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classifying matter in terms of light

Matter can be classified on the basis of how light interacts with it. Matter may be transparent, translucent, or opaque. Each type of matter is illustrated in Figure 22.3. Transparent matter is matter that transmits light without scattering it. Examples of transparent matter include air, pure water, and clear glass. You can see clearly through a transparent object, such as the revolving glass doors in the figure, because all the light passes straight through it. Translucent matter is matter that transmits but scatters light. Light passes through a translucent object but you cannot see clearly through the object because the light is scattered in all directions. The frosted glass doors in the figure are translucent. Opaque matter is matter that does not let any light pass through it. Matter may be opaque because it absorbs light, reflects light, or does both. Examples of opaque objects are solid wooden doors and glass mirrors. A wooden door absorbs most of the light that strikes it and reflects just a few wavelengths of visible light. A mirror, which is a sheet of glass with a shiny metal coating on the back, reflects all the light that strikes it.

artificial lights

The classroom in Figure 22.1 has artificial light sources in addition to natural sunlight. There are fluorescent lights on the ceiling of the room. There are also projectors on the ceiling that are shining light on screens. In these and most other artificial light sources, electricity provides the energy and some type of light bulb converts the electrical energy to visible light. How a light bulb produces visible light varies by type of bulb, as you can see in Table 22.1. Incandescent light bulbs, which produce light by incandescence, give off a lot of heat as well as light, so they waste energy. Other light bulbs produce light by luminescence, so they produce little if any heat. These light bulbs use energy more efficiently. Which types of light bulbs do you use? Type of Light Bulb Incandescent Light Description An incandescent light bulb produces visible light by incandescence. The bulb contains a thin wire filament made of tungsten. When electric current passes through the filament, it gets extremely hot and glows. You can learn more about incandescent light bulbs at the URL below. Fluorescent Light A fluorescent light bulb produces visible light by flu- orescence. The bulb contains mercury gas that gives off ultraviolet light when electricity passes through it. The inside of the bulb is coated with a substance called phosphor. The phosphor absorbs the ultraviolet light and then gives off most of the energy as visible light. You can learn more about fluorescent light bulbs at this URL: http://science.discovery.com/videos/deco Type of Light Bulb Neon Light Vapor Light LED Light Description A neon light produces visible light by electrolumines- cence. The bulb is a glass tube that contains the noble gas neon. When electricity passes through the gas, it excites electrons of neon atoms, causing them to give off visible light. Neon produces red light. Other noble gases are also used in lights, and they produce light of different colors. For example, krypton produces violet light, and argon produces blue light. A vapor light produces visible light by electrolumi- nescence. The bulb contains a small amount of solid sodium or mercury as well as a mixture of neon and argon gases. When an electric current passes through the gases, it causes the solid sodium or mercury to change to a gas and emit visible light. Sodium vapor lights, like these streetlights, produce yellowish light. Mercury vapor lights produce bluish light. Vapor lights are very bright and energy efficient. The bulbs are also long lasting. LED stands for light-emitting diode. This type of light contains a material, called a semi-conductor, which gives off visible light when a current runs through it. LED lights are used for traffic lights and indicator lights on computers, cars, and many other devices. This type of light is very reliable and durable.

light and matter

When visible light strikes matter, it interacts with it. How light interacts with matter depends on the type of matter.

how light interacts with matter

Light may interact with matter in several ways. Light may be reflected by matter. Reflected light bounces back when it strikes matter. Reflection of light is similar to reflection of sound waves. You can read more about reflection of light later on in this chapter in the lesson Optics. Light may be refracted by matter. The light is bent when it passes from one type of matter to another. Refraction of light is similar to refraction of sound waves. You can also read more about refraction of light in the lesson Optics. Light may pass through matter. This is called transmission of light. As light is transmitted, it may be scattered by particles of matter and spread out in all directions. This is called scattering of light. Light may be absorbed by matter. This is called absorption of light. When light is absorbed, it doesnt reflect from or pass through matter. Instead, its energy is transferred to particles of matter, which may increase the temperature of matter.

sources of visible light

Look at the classroom in Figure 22.1. It has several sources of visible light. One source of visible light is the sun. Sunlight enters the classroom through the windows. The sun provides virtually all of the visible light that living things need. Visible light travels across space from the sun to Earth in electromagnetic waves. But how does the sun produce light? Read on to find out.

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how visible light is produced

The sun and other stars produce light because they are so hot. They glow with light due to their extremely high temperatures. This way of producing light is called incandescence. Some objects produce light without becoming very hot. They generate light through chemical reactions or other processes. Producing light without heat is called luminescence. Objects that produce light by luminescence are said to be luminous. Luminescence, in turn, can occur in different ways: One type of luminescence is called fluorescence. In this process, a substance absorbs shorter-wavelength light, such as ultraviolet light, and then gives off light in the visible range of wavelengths. Certain minerals produce light in this way. Another type of luminescence is called electroluminescence. In this process, a substance gives off light when an electric current runs through it. Some gases produce light in this way. A third type of luminescence is called bioluminescence. This is the production of light by living things as a result of chemical reactions. Examples of bioluminescent organisms are pictured in Figure 22.2. You can learn more about bioluminescence in the video at this URL: Many other objects appear to produce their own light, but they actually just reflect light from another source. The moon is a good example. It appears to glow in the sky from its own light, but in reality it is just reflecting light from the sun. Objects like the moon that are lit up by another source of light are said to be illuminated. Everything you can see that doesnt produce its own light is illuminated.

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kqed color by nano

Artist Kate Nichols longed to paint with the iridescent colors of butterfly wings, but no such pigments existed. So she became the first artist-in-residence at Lawrence Berkeley National Laboratory to synthesize nanoparticles and incorporate them into her artwork. From the laboratory to the studio, see how Kate uses the phenomenon known as "structural color" to transform nanotechnology into creativity. For more information on using nanoparticles to create colors, see http://science.kqed.org/quest/video/science-on-the-spot-color-by-nano-the-art-of-kate-nichols/ . MEDIA Click image to the left or use the URL below. URL:

colors of light

Visible light consists of a range of wavelengths. The wavelength of visible light determines the color that the light appears. As you can see in Figure 22.4, light with the longest wavelength appears red, and light with the shortest wavelength appears violet. In between is a continuum of all the other colors of light. Only a few colors of light are represented in the figure.

instructional diagrams

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questions

referring to matter that allows all visible light to pass through

a. incandescence

b. translucent

c. pigment

d. luminescence

e. transmission

-->  f. transparent

g. opaque

Light bulbs that produce light by electroluminescence include

a. neon light bulbs.

b. vapor light bulbs.

c. LED light bulbs.

-->  d. all of the above

production of visible light in a way that does not require high temperatures

a. incandescence

b. translucent

c. pigment

-->  d. luminescence

e. transmission

f. transparent

g. opaque

You can see clearly through an object that is transparent because all of the light that strikes the object is

-->  a. transmitted.

b. reflected.

c. refracted.

d. absorbe

referring to matter that does not allow visible light to pass through it

a. incandescence

b. translucent

c. pigment

d. luminescence

e. transmission

f. transparent

-->  g. opaque

The shortest wavelength of visible light appears to the human eye as the color

a. red.

-->  b. violet.

c. yellow.

d. magenta.

Why does a blackboard appear black?

a. It reflects all wavelengths of visible light.

-->  b. It absorbs all wavelengths of visible light.

c. It reflects only black wavelengths of visible light.

d. It absorbs only black wavelengths of visible light.

production of visible light by an object that is so hot it glows

-->  a. incandescence

b. translucent

c. pigment

d. luminescence

e. transmission

f. transparent

g. opaque

passage of light through matter

a. incandescence

b. translucent

c. pigment

d. luminescence

-->  e. transmission

f. transparent

g. opaque

Which of the following colors is a primary pigment color?

a. red

b. blue

-->  c. cyan

d. green

referring to matter that transmits but scatters visible light

a. incandescence

-->  b. translucent

c. pigment

d. luminescence

e. transmission

f. transparent

g. opaque

substance that colors materials by reflecting light of certain wavelengths and absorbing light of other

a. incandescence

b. translucent

-->  c. pigment

d. luminescence

e. transmission

f. transparent

g. opaque

A luminous object is an object that appears to glow because it is reflecting light from another source.

a. true

-->  b. false

Some minerals produce visible light when they absorb ultraviolet light.

-->  a. true

b. false

A neon light produces violet or blue light.

a. true

-->  b. false

An object that reflects all the light that strikes it is opaque.

-->  a. true

b. false

Fireflies glow with visible light because of chemical reactions.

-->  a. true

b. false

The sun and other stars produce visible light by

a. fluorescence.

b. luminescence.

-->  c. incandescence.

d. electroluminescence.

Jellyfish and fireflies produce light as a result of

a. high temperatures.

-->  b. chemical reactions.

c. absorption of ultraviolet light.

d. reflection of light from other sources.

Which type of light bulb produces visible light by electroluminescence?

a. incandescent light bulb

b. vapor light bulb

c. neon light bulb

-->  d. two of the above

An example of opaque matter is a

a. clear glass window.

b. wooden door.

c. mirror.

-->  d. two of the above

Light with the longest wavelength appears

-->  a. red.

b. blue.

c. green.

d. violet.

A prism separates light into different colors by

a. reflection.

-->  b. refraction.

c. scattering.

d. transmission.

If only green light strikes a blue object, the object appears

a. green.

b. blue.

-->  c. black.

d. white.

All plants use visible light to make food by photosynthesis.

-->  a. true

b. false

The moon is an example of a luminescent object.

a. true

-->  b. false

The filament of an incandescent light bulb glows because it gets extremely hot.

-->  a. true

b. false

An LED light produces visible light by fluorescence.

a. true

-->  b. false

You can see clearly through an object that is translucent.

a. true

-->  b. false

A rainbow occurs because raindrops separate light into its different wavelengths.

-->  a. true

b. false

An apple appears red because it absorbs only red light.

a. true

-->  b. false

The bluish green color called cyan is a secondary color of light.

-->  a. true

b. false

Combining red, green, and blue light produces light that appears to be black.

a. true

-->  b. false

The primary colors of pigments are the same as the primary colors of light.

a. true

-->  b. false

diagram questions

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