Download Chapter 27 Color

Chapter 27 Color
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04/14/04
Selective reflection
Selective transmission
Mixing colored light
Why the sky is blue
Why sunsets are red
Why clouds are white
Why water is greenish blue
Dr. Jie Zou PHY 3050G
Department of Physics
1
Color
• The colors we see depend on the frequency or wavelength of the
light we see.
• Lights of different frequencies or wavelengths are perceived as
different colors.
– The lowest-frequency light we detect appears to most people as the color
red, and the highest frequency as violet.
• Between them range the infinite number of hues that make up the
color spectrum of the rainbow.
– These hues are grouped into the seven colors of red, orange, yellow, green,
blue, indigo, and violet. These colors together appear white.
– The white light from the sun is a composite of all visible frequencies.
04/14/04
Dr. Jie Zou PHY 3050G
Department of Physics
2
Selective reflection
• Selective reflection: Objects reflect
specific colors of light. They reflect only
part of the light that is incident upon
them, the part that gives them their color.
– For example, the atoms of the petals of a red rose
absorb all visible light except red, which they
reflect, and that is why the petals appear red.
– An object that reflects light of all the visible
frequencies, such as a white page, is the same color
as the light that shines on it.
– If a material absorbs all the light that shines on it, it
reflects none and hence it appears black.
• Color depends on the light source since
an object can reflect only those
frequencies present in the illuminating
light.
04/14/04
Dr. Jie Zou PHY 3050G
Department of Physics
3
Selective transmission
• The color of a transparent object depends on the color of the
light it transmits.
• Selective transmission: Objects selectively absorb light of
certain frequencies and selectively transmit others.
– Ordinary window glass is colorless because it transmits light of all
visible frequencies equally well.
04/14/04
Dr. Jie Zou PHY 3050G
Department of Physics
4
Mixing colored light
• Radiation curve of sunlight: the
graphical distribution of brightness
versus frequency.
– The intensity of light from the Sun varies with
frequency, being most intense in the yellow-green
part of the spectrum.
• All the colors combine make white. The
perception of white by our eyes also can
result from the combination of only red,
green, and blue light.
• Red, green, and blue are also called the
additive primary colors.
– Various amounts of red, green, and blue add to
produce any color in the visible spectrum.
04/14/04
Dr. Jie Zou PHY 3050G
Department of Physics
5
Why the sky is blue
• Scattering: when light is re-emitted
in all direction, we say the light is
scattered.
• Why the sky is blue:
When the air is full of particles larger
than molecules, lower-frequency light
is also scattered, which add to give a
whitish sky.
Why does the sky appear deeper blue
after a heavy rainstorm?
04/14/04
– Nitrogen and oxygen molecules in the
atmosphere scatter high-frequency light
more strongly than low-frequency ones.
– Since our eyes are not very sensitive to
violet light, the blue scattered light is what
predominates in our vision, and we see a
blue sky.
• The blue of the sky varies in
different places under different
conditions.
– Dust and other particles larger than
molecules also scatter light of lower
frequencies strongly, which makes the sky
appear whitish.
Dr. Jie Zou PHY 3050G
Department of Physics
6
Why sunsets are red
•
Light of lower frequencies, such as
red, orange, and yellow is the least
scattered by the atmosphere, and thus
is better transmitted through the air.
•
Red, which is scattered the least and
thus transmitted the most, passes
through more atmosphere than any
other color.
•
Sunlight travels through more
atmosphere at sunset, and thus more
time to scatter all the higherfrequency components of the light,
leaving the Sun look redder, going
from yellow to orange and finally to a
red-orange at sunset.
04/14/04
Dr. Jie Zou PHY 3050G
Department of Physics
7
Why clouds are white
• Water droplets in a variety of sizes
make up clouds.
• The different-size droplets produce
a variety of scattered frequencies.
– The tiniest scatter more blue than
other colors.
– Slightly larger droplets scatter slightly
more higher frequencies, say green.
– Still larger droplets scatter more red.
• The overall result is a white cloudAll the colors combined make
white.
04/14/04
Dr. Jie Zou PHY 3050G
Department of Physics
8
Why water is greenish blue
Water is cyan because it absorbs red light. The
froth in the waves is white because, like clouds,
it is composed of a variety of tiny water droplets
that scatter all the visible frequencies.
04/14/04
• While the sky is blue
because blue is strongly
scattered by molecules in
the atmosphere, water is
bluish green because red is
absorbed by molecules in
the water, hence leaving
the water to have a cyan
color.
Dr. Jie Zou PHY 3050G
Department of Physics
9
What is color?
• The colors of things depend on which colors are scattered
or reflected by molecules and also on which colors are
absorbed by molecules.
• The color we see is not in the world around us-the color is
in our heads.
• Electromagnetic waves of different frequencies (or
wavelengths) stimulate the sensation of color when the
vibrations interact with the cone-shaped receiving antennae
in the retina of our eyes.
04/14/04
Dr. Jie Zou PHY 3050G
Department of Physics
10
Let’s do some examples
• 1) Color depends on what characteristic of light?
– A) Its frequency. B) Its wavelength. C) Both of these. D) Neither of these.
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2) What color of light is transmitted by a piece of blue glass?
– A) Red. B) White. C) Blue. D) Yellow. E) Orange.
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3). The sky is blue because air molecules in the sky act as tiny
– A) mirrors which reflect only blue light. B) resonators which scatter blue light. C)
sources of white light. D) prisms. E) none of these.
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4). Red sunsets are due to lower frequencies of light that
– A) are scattered from larger particles in the air.
– B) are reflected by clouds and relatively large particles in the air.
– C) survive being scattered in the air.
– D) appear reddish orange to the eye.
• 5) The whiteness of clouds is evidence in the clouds for a variety
of
– A) particle sizes. B) molecules. C) “seeds” upon which condensation of cloud
material forms. D) light intensities. E) water prisms.
04/14/04
Dr. Jie Zou PHY 3050G
Department of Physics
11
Homework 12 (04/14/04)
• Chapter 27, P. 530, Exercises: 9, 10, 27, 36.
04/14/04
Dr. Jie Zou PHY 3050G
Department of Physics
12