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COLOR

?2006 Honors Physics II 1

Objectives

Define white light and the components of light. Explain the significance of ROY G BIV.

Explain the purpose of a diffraction grating and use it to interpret spectra.

Define black.

List the primary and secondary colors of light. Use the color wheel of light to explain color by addition. Explain selective transmission and selective reflection. Explain why the sky is blue.

Explain why the ocean appears blue and is actually cyan in color. Explain why sunsets often appear red.

Compare and contrast pigments to colored light.

Key Terms

spectrum primary color of light pigment

ROY G BIV secondary color of light primary color of pigment prism color wheel of light secondary color of pigment black color by addition color wheel of pigment diffraction grating selective reflection color by subtraction spectroscope selective transmission scattering

2 Honors Physics II ?2006

The Color of Light

Demonstrations and Notes

Place a piece of opaque construction paper over the face of a flashlight. Cut a narrow slit

in the center of the paper slightly less than the face diameter. Shine light from the

flashlight through a prism, projecting the light on a screen. Observe the light leaving the

prism, focusing on the order the colors are arranged.

spectrum visible light; white light; range on electromagnetic spectrum between infrared and 15ultraviolet; wavelength range between 400 nm to 700 nm; frequency around 10 hertz; represents

less than one percent of the electromagnetic spectrum; the only thing we can see.

ROY G BIV acronym for the colors of the visible spectrum; red, orange, yellow, green, blue, indigo, and violet; red ~ 700 nm, violet ~ 400 nm.

Indigo and violet are sometimes combined into one color violet. When Newton studied light

and color, he tried to relate color to sound. Since there are seven notes in a scale of music, Newton thought there should be seven colors in the spectrum.

prism triangular piece of glass used to separate white light into its component colors.

Since different colors of light have varying frequencies and wavelengths, every color of the spectrum travels through glass at a different speed. This allows a prism to slow down white light and separate it into the spectrum. This can be compared to crossing a room at a crowded party.

?2006 Honors Physics II 3

People in the room are like the atoms of glass. The people crossing the room represents the various colors of light. Since we are looking for the fastest color, we only need to compare the two extremes: red and violet. Think of red (large wavelength and low frequency) as a mellow person; they walk slow, take their time, and are never in a hurry. Violet (small wavelength and high frequency) is a hyper person; drinks too much coffee, walks fast, and appears disorganized.

To determine who will cross the room first, look at their approaches. Red walks slowly and takes his time. People can see him coming, move out of his way, and let him pass with minimal interaction. Violet is hyper and in a hurry. People don’t see him coming; he runs up to them and bounces off. After each bounce, he is a bit disoriented and hits someone else. Eventually he’ll get across the room, but each interaction costs him time. Therefore, red light travels the

fastest through glass and violet is the slowest. The color on top of a rainbow is red because it moves fastest. Violet is on the bottom because it moves the slowest.

black not a color; absence of color; shadow.

A black light or ultraviolet lamp is almost the same as a fluorescent light. The differences are that ultraviolet lights lack the white phosphor coating and are made with dark blue glass. Fluorescent tubes contain a near vacuum with mercury vapor inside. When excited electrically, the mercury gas gives off visible green, blue, and violet light as well as invisible ultraviolet frequencies. If black light tubes were made from clear glass, the bright blue/green glow would light up the room. Because of this, black lights are made with dyed glass which blocks most of the visible light but passes the ultraviolet. If a perfect dye were available, black light tubes would not glow purple when on; they would look totally black. However, the dye in the glass does let a little visible violet light through as well as passing a lot of invisible ultraviolet rays. 4 Honors Physics II ?2006

Light a light bulb. Look at the light through a diffraction grating. Observe the diffraction

of light into the spectrum of colors, focusing on the outer region of the diffraction grating.

diffraction grating material containing many parallel lines very closely spaced used to separate colors of light due to interference; acts as multiple slits that cause overlapping diffraction.

Look out the window through the spectroscope. Observe the spectrum of color created in

the spectroscope.

spectroscope device containing a diffraction grating used to analyze the spectrum of materials.

Applications of a Diffraction Grating

Activity

Give each person a diffraction grating. Light various vapor bulbs (carbon dioxide, helium,

hydrogen, mercury, etc.). Observe each of the gasses through the diffraction grating.

Describe what is seen through the diffraction grating for each gas, relating the color of the

vapor lamp to the diffracted light. Since white light contains all of the colors of the

spectrum, the more white the light, the more colors of the spectrum that are seen.

?2006 Honors Physics II 5

Summary Review The Color of Light and Applications of a Diffraction Grating 15 hertz. ; Visible light ranges from 400 nm to 700 nm at a frequency of around 10

; The colors of visible light include red, orange, yellow, green, blue, indigo, and violet, or

ROY G BIV.

; Violet has a high frequency and small wavelength. Red has a low frequency and large

wavelength. This allows red to travel faster when separated from white light. ; Black is not a color of light, it is the absence of color.

; A diffraction grating or prism can be used to separate white light into its component colors.

Properties of the Color of Light

Demonstrations and Notes

White light is the combination of all colors; red, orange, yellow, green, blue, and violet.. These colors come from the sun, which emits mostly yellow and green light. The average of all the colors of the spectrum are red, green, and blue. This is easily seen when looking at a computer’s monitor set up.

Open the Color Correction section of the Display Properties of any PC. Observe the

colors available to adjust the color properties.

Only red, green, and blue are available to adjust because all colors can be created from red, green, and blue. By varying the intensity of the colors, 256 different colors can be created. When fine tuning the intensity, 32 million colors are available. The maximum number of colors available is 64 million. This only applies to light and NOT pigments.

Red, green, and blue are considered the average of white light. Red is the result of red light and orange light. Green is the result of yellow light, green light, and some blue light. Blue is the result of the rest of the blue light, indigo light, and violet light.

primary color of light average colors of sunlight or white light; colors of light that when added together produce white light; red, green, and blue.

secondary color of light result of adding any two primary colors of light; color of light, that when added to its primary color of light, produces white light; yellow, cyan, and magenta. color wheel of light used to show how various colors of light are created.

6 Honors Physics II ?2006

color by addition process used to determine the resultant color of combined light; the adding of colored light.

Use the Color Box to show how various colors of light combine to make white light.

To determine the resultant color of light, reduce all colors of light used to a primary color of light, then add them up. For example, yellow light plus blue light yields white light. This is because yellow light can be thought of as red light and green light. When yellow light is then added to blue light, all three primary colors of light are present. The result of red light, green light, and blue light is white light. Yellow light plus magenta light produces pink light. This is because yellow light can be thought of as red light and green light. Yellow light is then added to magenta light. Magenta light can be thought of as blue light and red light. This means all three primary colors of light are present, which produces white light. However, there is extra red light. White light and red light yields pink light.

Observe a solid colored object. Determine why the object is seen as a specific color.

selective reflection opaque objects reflect the color seen and absorb all other colors.

Observe a transparent colored prism. Determine why the prism appears blue. ?2006 Honors Physics II 7

selective transmission transparent objects transmit the color seen and absorb all other colors.

Translucent objects use both reflection and transmission depending on their composition.

Properties of the Color of Pigment

Demonstrations and Notes

Observe a solid colored object. Determine why the object appears the color seen.

pigment materials that selectively absorb colored light; some reemit absorbed light, like neon and fluorescent paint; most pigments are dyes, paints, or ink.

Pigments are the opposite of light. In general, light is seen due to reflection and pigments exist due to absorption. Pigments absorb the colors not seen and reflect the colors seen. The color wheel for pigments include yellow, magenta, and cyan as the primary colors of pigment and red, blue, and green as the secondary colors of pigment. Note that this is the opposite of the color wheel of light. This is because pigments that reflect only one color of light are the primary pigments and pigments that reflect two colors of light are the secondary pigments.

primary color of pigment base colors of all pigments (all pigments are made from these colors); colors of pigment that when mixed produce black pigment; yellow, magenta, and cyan. secondary color of pigment result of mixing any two primary colors of pigment; pigment, that when added to its primary pigment, produces black pigment; red, blue, and green. color wheel of pigment used to show how various colors of pigment are created.

8 Honors Physics II ?2006

Use the three primary colors of pigment to show how other colors of pigment are made.

We often say yellow and blue pigments make green pigment. However it is more accurate to say yellow and cyan pigments make green pigment. Pigments are made through color by subtraction.

color by subtraction process used to determine the resultant color of combined pigments; the subtraction of pigments.

Summary Review Properties of the Color of Light and Properties of the Color of Pigment ; The average of white light consists of red light, green light, and blue light. ; The primary colors of light include red, green, and blue.

; The secondary colors of light are yellow, cyan, and magenta.

; The color wheel of light shows how colors of light are produced using color by addition. ; The color of an opaque object is seen due to selective reflection.

; The color of a transparent object is seen due to selective transmission. ; A pigment is the opposite of light; it is the color of a material object due to absorption. ; The primary colors of pigment include yellow, magenta, and cyan.

; The secondary colors of pigment are red, blue, and green.

; The color wheel of pigment shows how pigments are produced using color by subtraction.

?2006 Honors Physics II 9

Effects of Color

Discussion and Notes

Why is the sky blue? Recall the description of red as an easy going, slow mover and violet as a hyper, fast mover; green is somewhere in between. The atmosphere contains mostly oxygen (O) and nitrogen (N), which are larger than light rays. As the different colors separate 22

through the atmosphere, red has little chance of hitting any nitrogen or oxygen. Green has a greater chance of hitting the oxygen and nitrogen whereas blue, with its high frequency, has the largest probability of scattering. The scattering of mostly blue makes the sky appear blue.

scattering the reflection of light in the air.

After it rains, the atmosphere has less oxygen and nitrogen throughout. Compare this to spraying a room full of people (oxygen and nitrogen) with a fire hose. Most people will be forced out of the room while only few will be left behind. With less things in the air, there are even less chances of scattering. This makes the sky appear even more blue.

10 Honors Physics II ?2006

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