Physics - Light and Color

Lecture Outline

Chapter 27: Color

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This lecture will help you understand:

• Color in Our World
• Selective Reflection
• Selective Transmission
• Mixing Colored Light
• Mixing Colored Pigments
• Why the Sky Is Blue
• Why Sunsets Are Red
• Why Clouds Are White
• Why Water Is Greenish Blue

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Color in Our World

• Color

– Physiological experience
– In the eye of the beholder

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Color in Our World, Continued

• Color we see depends on frequency of light.

– Lowest frequency—perceived as red
– In between lowest and highest

frequency—perceived as colors of the
rainbow (red, orange, yellow, green, blue,
indigo, violet)

– Highest frequency—perceived as violet
– Beyond violet, invisible ultraviolet (UV)

Multiple Choice

The highest frequency of light visible to humans is

• blue

• indigo

• violet

• ultraviolet

Multiple Choice

What can the human eye NOT see?

• Infrared radiation

• ultraviolet radiation

• Both A and B

• Neither A and B

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Selective Reflection

• Selective reflection

– We see the color of a rose by the light it

reflects.

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Selective Reflection, Continued

• Objects reflect light of some frequencies and absorb the

rest.
– Rose petals absorb most of the light and reflect red.
– Objects that absorb light and reflect none appear

black.

– Objects can reflect only those frequencies present in

the illuminating light.

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Selective Transmission

• Color of transparent object depends on color of

light it transmits.

– Colored glass is warmed due to the energy of

absorbed light illuminating the glass.

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Mixing Colored Light

• Mixed colored lights

– Distribution of solar frequencies is uneven.

• Most intense in yellow-green portion (where our

eyes are most sensitive)

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Mixing Colored Light, Continued

• Radiation curve divides into three regions that

match the color receptors in our eyes.

Open Ended

What is the most common color used in safety apparel and explain why that is.

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Mixing Colored Light, Continued-1

• Additive primary colors:

– Red, green, and blue
– Produce any color in the spectrum

Multiple Choice

When the color yellow is seen on your TV screen the colors being activated on the screen are

• mainly yellow

• blue and red

• green and yellow

• red and green

Multiple Choice

A blue object will appear black when it's illuminated with

• blue light

• cyan light

• yellow light

• magenta light

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Mixing Colored Light, Continued-2

• Subtractive primary colors

– Combination of two of the three additive

primary colors:

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

Multiple Choice

The subtractive colors of light are

• red, blue, and green

• cyan, red, and yellow

• magenta, green, and, blue

• magenta, cyan, and yellow

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Mixing Colored Light, Continued-3

• The shadows of the golf ball are subtractive.

– Magenta (opposite of green)
– Cyan (opposite of red)
– Yellow (opposite of blue)

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Mixing Colored Light, Continued-4

• Subtractive primaries are complementary to additive

primaries.
– magenta + green = white = red + blue + green
– yellow + blue = white + red + green + blue
– Example: color printing

Multiple Choice

A red rose will not appear red when illuminated only with

• red light

• orange light

• white light

• cyan light

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Mixing Colored Pigments

•Only three colors of ink (plus black) are used to print color
photographs—(a) magenta, (b) yellow, (c) cyan, which when
combined produce the colors shown in (d). The addition of black (e)
produces the finished result (f).

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Mixing Colored Pigments, Continued

•The subtractive primary colors
are cyan, yellow, and magenta.

•When white light passes
through overlapping sheets of
these colors, light of all
frequencies is blocked
(subtracted) and we have
black.

•Where only cyan and yellow
overlap, light of all frequencies
except green is subtracted.

•Various proportions of cyan,
yellow, and magenta dyes will
produce nearly any color in the
spectrum.

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Why the Sky Is Blue

• Why the sky is blue

– Results of selective scattering of smaller particles

than the wavelength of incident light and resonances
at frequencies higher than scattered light

– The tinier the particle, the higher the frequency of

light it will re-emit.

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Why the Sky Is Blue, Continued

• Why the sky is blue (continued)

– Due to selective scattering
– Blue scattered light predominates in our vision.
– Varies in different locations under various conditions:

• Clear dry day—much deeper blue sky
• Clear, humid day—beautiful blue sky
• Lots of dust particles and larger molecules than nitrogen and

oxygen in the atmosphere—less blue sky with whitish
appearance

• After heavy rainstorm (washing away of airborne

particles)—deeper blue sky

Multiple Choice

A white sky is evidence that the atmosphere contains

• predominantly small particles

• predominantly large particles

• a mixture of sized particles

• pollutants

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Why Sunsets Are Red

• Light that is least scattered is light of low

frequencies, which best travel through air.
– Red
– Orange
– Yellow

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Why Sunsets Are Red
CHECK YOUR NEIGHBOR
A variety of sunset colors is evidence for a variety
of

A.

elements in the Sun.

B.

apparent atmosphere thickness.

C.

atmospheric particles.

D.

primary colors.

Multiple Choice

A variety of sunset colors is evidence for a variety of

• elements in the Sun

• apparent atmospheric thickness

• atmospheric particles

• primary colors

Multiple Choice

If molecules in the sky scattered orange light instead of blue light, sunsets would be

• orange

• yellow

• green

• blue

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Why Sunsets Are Red
CHECK YOUR ANSWER, Continued
If molecules in the sky scattered orange light instead of
blue light, sunsets would be

D.

blue.

Explanation:
Of the colors listed, blue is closest to being the
complementary color of orange.

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Why Clouds Are White

• Clouds

– Clusters of various sizes of water droplets

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Why Clouds Are White, Continued

• Size of clusters determines scattered cloud

color.
– Tiny clusters produce bluish clouds.
– Slightly large clusters produce greenish

clouds.

– Larger clusters produce reddish clouds.
– Overall result is white clouds.
– Slightly larger clusters produce a deep gray.
– Still larger clusters produce raindrops.

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Why Water Is Greenish Blue

•Water molecules resonate somewhat in the visible red, which
causes red light to be a little more strongly absorbed in water than
blue light.

•Red light is reduced to one-quarter of its initial brightness by 15
meters of water. There is very little red light in the sunlight that
penetrates below 30 meters of water.

•When red is removed from white light, the complementary color of
red remains: cyan—a bluish-green color.

Multiple Choice

Water molecules resonate with _____ light which causes it to be absorbed, causing ______ light to get reflected.

• blue, red

• red, green

• green, blue

• red, blue

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Why Water Is Greenish Blue, Continued

• The intriguingly vivid blue

of lakes in the Canadian
Rockies is due to
scattering.

• The lakes are fed by runoff

from melting glaciers that
contain fine particles of
silt, called rock flour, which
remain suspended in the
water.

• Light scatters from these

Multiple Choice

Moraine Lake is in the Province of

• Canada

• British Columbia

• Alberta

• Toronto

Replace this with a header

Moraine Lake

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