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Chapter 16: Light and Color

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CPO Science
Foundations of Physics
Unit 6, Chapter 16
Unit 6: Light and Optics
Chapter 16 Light and Color
 16.1 Properties and Sources of Light
 16.2 Color and Vision
 16.3 Photons and Atoms
Chapter 16 Objectives
1. Describe at least five properties of light.
2. Describe the meaning of the term “intensity.”
3. Use the speed of light to calculate the time or
distance traveled by light.
4. Explain how we perceive color in terms of the three
primary colors.
5. Explain the difference between the additive and
subtractive color processes.
6. Arrange the colors of light in order of increasing
energy, starting with red.
7. Describe light in terms of photons, energy, and color.
Chapter 16 Vocabulary Terms
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reflection
refraction
black
fluorescence
intensity
color
blue
light ray
CMYK color
ultraviolet
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infrared
photon
RBG color
photoluminescence
additive color
white
red
green
spherical pattern
cyan
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magenta
yellow
pigment
speed of light (c)
incandescence
pixel
rod cell
cone cell
subtractive color
photoreceptor
16.1 Properties and Sources of Light
Key Question:
What are some useful
properties of light?
*Students read Section 16.1 AFTER Investigation 16.1
16.1 Properties and Sources of Light
 Light travels almost unimaginably fast and far.
 Light carries energy and information.
 Light travels in straight lines.
 Light bounces and bends when it comes in contact with
objects.
 Light has color.
 Light has different intensities, it can be bright or dim.
16.1 Electric Light
 The process of making light with heat is called
incandescence.
 Incandescent bulbs generate light when electricity
passes through a thin piece of metal wire called a
filament.
 The filament heats up and
gives off light.
16.1 Electric Light
 The other common kind of electric light is the
fluorescent bulb.
 Fluorescent bulbs
convert electricity
directly to light
without generating a
lot of heat.
 Fluorescent bulbs
use high-voltage
electricity to
energize atoms of
gas that fill the bulb.
16.1 Light carries energy and power
 Light is a form of energy that travels.
 The intensity of light is the amount of energy per
second falling on a surface.
 Most light sources distribute their light equally in
all directions, making a spherical pattern.
 Because light spreads out in a sphere, the
intensity decreases the farther you get from the
source.
16.1 Light intensity
 The intensity of light from a small source follows an
inverse square law because its intensity diminishes
as the square of the distance.
16.1 Light carries information
 The fiber-optic networks you read about are
pipelines for information carried by light.
16.1 Light carries information
 In some cities, a fiberoptic cable comes
directly into homes
and apartments
carrying telephone,
television, and
Internet signals.
16.1 The speed of light
 The speed at which light travels through air is
approximately 300 million meters per second.
 Light travels almost a million times faster than
sound.
16.1 The speed of light
 The speed of light is so important in physics that it
is given its own symbol, a lower case c.
 The best accepted experimental measurement for
the speed of light in air is 299,792,500 m/sec.
 For most purposes, we do not need to be this
accurate and may use a value for c of 3 Г— 108
m/sec.
16.1 Calculate time
 Calculate the time it takes light and sound to
travel the distance of one mile, which is 1,609
meters.
16.1 Reflection and refraction
 When light moves through a material it travels in
straight lines.
 When light rays travel from one material to another,
the rays may reflect.
 The light that appears to bounce off the surface of an
object is shown by a reflected ray.
16.1 Reflection and refraction
 Objects that are in front of a mirror appear as if
they are behind the mirror.
 This is because light rays are reflected by the
mirror.
 Your brain perceives the light as if it always
traveled in a straight line.
16.1 Reflection and refraction
 Another example of
refraction of light is the
twinkling of a star in the
night sky
 As starlight travels from
space into the Earth’s
atmosphere, the rays are
refracted.
 Since the atmosphere is
constantly changing, the
amount of refraction also
changes.
16.1 Reflection and refraction
 The light that bends as it crosses a surface into a
material refracts and is shown as a refracted ray.
16.2 Color and Vision
Key Question:
How do we see color?
*Students read Section 16.2
AFTER Investigation 16.2
16.2 Color and Vision
 When all the colors of the rainbow are combined, we
do not see any particular color.
 We see light without any color.
 We call this combination of all the colors of light
"white light".
16.2 Color and Vision
 We can think of different
colors of light like balls with
different kinetic energies.
 Blue light has a higher
energy than green light, like
the balls that make it into
the top window.
 Red light has the lowest
energy, like the balls that
can only make it to the
lowest window.
How the human eye sees color
 The retina in the back of
the eye contains
photoreceptors.
 These receptors release
chemical signals.
 Chemical signals travel to
the brain along the optic
nerve.
optic nerve
Photoreceptors in the eye
 Cones respond to
three colors: red,
green and blue.
 Rods detect intensity
of light: black, white,
shades of gray.
How we see colors
 Which chemical
signal gets sent
depends on how
much energy the
light has.
 If the brain gets a
signal from ONLY
green cones, we see
green.
16.2 How we see other colors
 The three color receptors in
the eye allow us to see
millions of different colors.
 The additive primary colors
are red, green, and blue.
 We don’t see everything
white because the strength
of the signal matters.
 All the different shades of
color we can see are made
by changing the proportions
of red, green, and blue.
16.2 How we see the color of things
When we see an object, the
light that reaches our
eyes can come from two
different processes:
1. The light can be emitted
directly from the object,
like a light bulb or glow
stick.
2. The light can come from
somewhere else, like the
sun, and we see the
objects by reflected light.
16.2 How we see the color of things
 Colored fabrics and paints
get color from a
subtractive process.
 Chemicals, known as
pigments, in the dyes and
paints absorb some colors
and allow the color you
actually see to be
reflected.
 Magenta, yellow, and cyan
are the three subtractive
primary colors.
16.2 Why are
plants green?
 Plants absorb energy
from light and convert it
to chemical energy in the
form of sugar (food for
the plant).
 Chlorophyll is an
important molecule that
absorbs blue and red
light.
16.2 How does a color TV work?
 Televisions give off light.
 To make color with a TV, you can use red, green, and
blue (RGB) directly.
 The screen is made of tiny
red, green, and blue dots.
 The dots are called pixels
and each pixel gives off
its own light.
 TV sets can mix the three
colors to get millions of
different colors.
16.3 Photons and Atoms
 Key Question:
How does light fit into the
atomic theory of
matter?
*Students read Section 16.3
AFTER Investigation 16.3
16.2 Photons and atoms
 Just like matter is made of tiny
particles called atoms, light
energy comes in tiny bundles
called photons.
 White light is a mixture of
photons with a wide range of
colors (energies).
 For a given temperature, the
atoms in a material have a range
of energy that goes from zero up
to a maximum that depends on
the temperature.
16.2 Photons and intensity
 Intensity measures power per unit area.
 There are two ways to make light of high intensity.
— One way is to have high- energy photons.
— A second way is to have a lot of photons even if they
are low-energy.
The number and
energy of photons
determine the
intensity of the light.
16.2 Photons and intensity
 The light from the flashlight
cannot energize phosphorus
atoms that your hand
blocks.
 These atoms will not glow
because they did not
receive any energy from
photons from the flashlight.
 The explanation is that each
phosphorus atom absorbs
(or emits) only one photon
at a time.
Application: Color Printing
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