You know that light is a form of energy that moves in a straight line.. The color that you see is the part of the light that does not get absorbed, but instead is refl ected in your eye.
Trang 1Scott Foresman Science 3.2
Nonfi ction Predict • Captions
• Diagrams
• Glossary
Light
ISBN 0-328-23522-9
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by Mary Beth Spann
Physical Sciences
Trang 2Picture Credits
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ISBN: 0-328-23522-9
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1 2 3 4 5 6 7 8 9 10 V010 13 12 11 10 09 08 07 06
Vocabulary
absorb
color
eye
light
mirror
opaque
refl ect
shadow
Extended Vocabulary
electromagnetic spectrum
microwaves photon
prism refract ultraviolet waves wavelength
Trang 3What You Already Know
You know many things about light and color You
know that light is a form of energy that moves in a
straight line Sunlight is the main source of Earth’s
energy Sunlight travels outward in straight lines in all
directions The light from a fl ashlight, lantern, lamp, or
any other light source also travels in straight lines
A beam of light continues to travel in a straight line
until it is blocked by another object Some objects, such
as an umbrella or the roof of a house, are opaque They
do not let light pass through them These opaque objects
will produce shadows in those spots where the light
cannot reach
Opaque objects cause
shadows to form when
light strikes them.
3
The eye is the part of the human body that is most sensitive to light energy In order to see something, the eye needs a source of light to shine on an object The light travels from the object to the eye and interacts with
it Then the brain interprets what is being seen Without light you would not be able to see
When light shines on an object, it can refl ect, or bounce off, the object A mirror, or fl at surface made
of shiny material, will refl ect more light than almost anything Rather than refl ecting light, many objects will absorb, or take in, light The color that you see is the
part of the light that does not get absorbed, but instead is refl ected in your eye
Light interacts with the environment in different ways
Read on to learn more about the way light travels You will learn about how the path of light determines the colors we see
You will even fi nd out about how rainbows are formed!
Trang 4Different Views
Of Color
How people think about color and color combinations
can depend on how they use the colors A printer who
works at a press may think of color as combinations
of colored dots A computer-based designer producing
images for the printer works with pixels of color on a
computer screen
An artist who works
with paints may view colors
as combinations of paint
pigments A
television-lighting engineer might
think about colors in terms
of combinations of red,
green, and blue lights A
scientist might think of
color as the way nature
transfers light energy
through space
None of these
viewpoints is wrong Color
can be thought of in many
different ways!
5
Light Is Energy
To scientists, light is energy Light energy travels at the incredible speed of 186, 282 miles per second That’s
almost six trillion miles traveled in one year Six trillion
is a big number Here’s an easier way to think of light’s speed: it can circle the Earth seven times in a second!
There are two ways to describe how light travels One way is to think of light as traveling in little packets of energy called photons You could think of these packets
of energy as “bullets” strung together in a row
The second way is to think of light as traveling in waves Some light waves are longer than other light waves The differences in the wavelengths of light allow us to see different colors in the world around us
Light acts like a wave and a packet of energy at the same time
Both ways of thinking about light are correct
Combinations of colored dots come together to form images at a printing press.
Trang 5cosmic rays
gamma
visible spectrum
6
Light and Color
A physicist named Max Planck
studied light, color, and energy
His ideas about light energy
changed physics forever Other
scientists have continued to
study light and color We now
know why light waves have
different colors
The length of a light wave,
or its wavelength, determines
its color Violet has the shortest
wavelengths that can be seen
with the human eye Red has the
longest wavelengths that can be seen
with the human eye All the other colors
have wavelengths that fall in between these two This
band of visible wavelengths makes up the color spectrum
Max Planck
shortest wavelength
s
infrared
radio waves, television
electric rays
7
The full range of light wavelengths, including the color spectrum, makes up a section of the electromagnetic spectrum This spectrum includes
X rays, gamma rays, ultraviolet (UV) rays, microwaves, and radio waves Radio waves have the longest
wavelength and the lowest amount of energy Gamma rays have the shortest wavelength and the highest amount of energy The other rays fall between radio waves and gamma rays on the electromagnetic spectrum
Several different units of measure are used in relation
to the electromagnetic spectrum The units of measure include the hertz, nanometer, and angström Hertz are used to measure the number of wavelengths over a period
of time Nanometers and angströms are used to measure wavelength distance
The electromagnetic spectrum includes light that you can see.
longest wavelength
Trang 6Refraction, Prisms,
And Rainbows
Light contains all the colors of the rainbow
Those colors are not apparent, however, unless
the light is separated into different colors How
do we separate light in order to see individual
colors?
The way we are able to see individual color
involves changing the medium, or material,
through which light travels The direction that
light travels usually changes as it passes from
one medium to another When a ray of light
passes from the air to water, it will bend, or
refract When white light refracts, it can split
apart into different wavelengths of visible color
Glass prisms demonstrate how white light
separates A prism is a transparent object that
is often three-sided When white light passes through a
prism, it is refracted Its different wavelengths refract at
different angles, forcing them to split apart When they
split apart, distinct colored light waves become visible
9
Raindrops act as tiny prisms that refract and refl ect white light As light enters a raindrop,
it changes mediums, from air
to water, and refracts The refraction separates the white light into different wavelengths
Each wavelength travels to the other side of the drop, where it
is then refl ected in the opposite direction Finally, each wave travels out of the drop and reenters the air When the light changes mediums, the light refracts again If enough light passes, it will refract, refl ect, and refract This will form a rainbow
Rainbows require both light and raindrops, so they usually form after a rainstorm To see a rainbow, the Sun has
to be behind you and the rainstorm has to be in front
of you As the Sun shines through the rain, the droplets refract and refl ect the white light back again Through this process, light separates into visible colors, treating observers to a light show in the sky!
Prisms like this one bend and separate light.
Trang 7Rainbow Colors
Sometimes people make up a code to help them
remember something One code used to talk about light
is the name Roy G Biv The letters stand for the colors
red, orange, yellow, green, blue, indigo, and violet The
colors of the color spectrum are always arranged in that
order They go from the longest wavelength (red) to
the shortest (violet) The colors of a rainbow are also
arranged in that order Secondary rainbows, which you
will read about later, are a little bit different
People usually think of rainbows in terms of only
those colors Rainbows actually consist of a whole range
of colors that blend from red to violet
There are actually more colors in a rainbow than what you see here.
11
What makes the “bow”?
The “bow” part of the word “rainbow” refers to the curved shape of the rainbow We don’t see the whole circle or disc of light color A full rainbow would take the shape of a circle, but we only see a semicircle That’s because the horizon blocks or absorbs the rest of the circle
In 1637, a scientist named René Descartes conducted a simple study of rainbow light He studied just one water droplet and learned how it acted when light fell on it Descartes observed that rainbows were not limited to appearing only after rainstorms
He observed that they appeared in the air near us whenever sunlight hit water droplets Descartes concluded that rainbows were formed from the way light acted
on the drops and how these changes appeared as they reached our eyes
Descartes studied how light enters the eye.
Trang 8Double Rainbows
Have you ever seen two rainbows at once? The
rainbow we usually see is called the primary, or fi rst,
rainbow and is made by light that has been refl ected once
inside of the raindrops When light exits the raindrops
at a different angle we can see a secondary rainbow A
secondary rainbow occurs when light refracts and then
refl ects twice in the rain droplets
The light that forms secondary rainbows is much
less intense than the light of primary rainbows The
arrangement of colors in a secondary rainbow is also
reversed compared to the primary rainbow Instead of
running from red to violet, they run from violet to red
Double rainbows form
two bands of color.
12
Different Types of Rainbows
Rainbows require relatively warm conditions in order
to form For a natural rainbow to appear, there needs to
be water and sunlight In cold weather, water droplets can freeze into ice particles Ice particles refract light into very interesting patterns, but not into rainbows
Rainbows are not limited to water droplets suspended
in the air They can form when substances such as oil and soap fl oat on the surface of the water Light passes from the air into the droplets of oil (or soap) First, it refracts
as it hits the new medium Then it refl ects back out as different wavelengths of visible color There has to be the right amount of oil or soap for this to occur Oil slicks or layers of soap that make rainbows are also called sheens
13
Trang 9What do you see?
There are other special types of rainbows beyond
those created by oil sheens and soap Red rainbows occur
during sunrise and sunset They are deeply and richly
colored rainbows that appear in mostly reds and yellows
Cloud rainbows occur when light passes through
tiny droplets of cloud mist These bows have light, pastel
colors A fogbow occurs in fog They are almost white
with a little bit of red and blue at the edges
A rainbow cone is the shape the rays of a rainbow
make as it reaches your eye Wheel rainbows occur when
clouds or dense rain showers shadow the light falling in
your rainbow cone When shadowed raindrops can no
longer send the rainbow’s rays toward your eye, you see
one or more dark shadows These shadows sometimes
resemble a wagon wheel
15
No two people ever see exactly the same rainbow
Scientists point out that as white light is split into wavelengths of visible light colors, each color reaches a defi nite end point—namely, the eye of the observer If the end point changes, so does the way the rainbow appears
Two separate observers cannot be in exactly the same point at the same time So they do not, and cannot, see the same rainbow Even each eye on the same observer sees its own rainbow!
There is only one way two people can enjoy the same rainbow They can look at a photo of one taken through the eye of a camera
A red rainbow hangs over a bay
in Homer, Alaska.
Trang 10electromagnetic all the waves of electromagnetic
radiation including the longest waves and visible light
microwaves electromagnetic waves with
wavelengths between infrared light and radio waves
photon a measure of electromagnetic
energy
prism a three-sided transparent object
that breaks light into rainbow colors
refract the bending of light as it passes
from one medium to another, changing its speed
ultraviolet rays light radiation with slightly
shorter wavelengths than the visible color violet
wavelength the distance in the line of a wave
from one point to the next corresponding point
X rays light with very short wavelengths
spectrum
1 What is light?
2 How does light travel?
3 What did Descartes discover about rainbows?
which you describe having seen a double rainbow
Include as many details as possible.
slanting through the clouds What type of rainbow do you predict might form?