Download Refraction, Lenses, and Prisms

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CHAPTER 16
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Sound and Light
SECTION
41 Refraction, Lenses, and Prisms
KEY IDEAS
As you read this section, keep these questions in mind:
• What happens to light when it passes from one
medium to another?
• How do lenses work?
• How can a prism separate white light into colors?
What Happens When Light Passes from One
Medium to Another?
Light, like all waves, travels in a straight line in a single
medium. However, remember that waves refract, or bend,
when they move from one medium to another. Like all
waves, light bends when it moves from one medium to
another. It does this because the speed of light is different in each medium.
The amount a light wave bends depends on the
relative speeds of light in each medium. If light slows
down when it enters a new medium, the light ray bends
toward the normal. If it speeds up when it enters a new
medium, it bends away from the normal.
For example, the speed of light is higher in air than in
glass. When light moves from air to glass, the ray bends
toward the normal. When light moves from glass to air,
the ray bends away from the normal, as shown below.
Glass
Air
Air
Glass
Normal
READING TOOLBOX
Compare and Contrast As
you read this section, make
a chart to compare and
contrast how light interacts
with mirrors, lenses, and
prisms.
READING CHECK
1. Explain Why does light
refract when it moves into a
different medium?
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Normal
When light moves from air into glass, When light moves from glass into
air, the light speeds up. The light ray
the light slows down. The light ray
bends away from the normal.
bends toward the normal.
2. Describe What happens
to the speed of light when
light moves from air into
glass?
REFRACTION AND VIRTUAL IMAGES
Remember that a virtual image is an image that
light rays do not pass through. Refraction of light, like
reflection off of a flat mirror, can produce virtual images.
The figure at the top of the next page shows an example
of this.
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Refraction, Lenses, and Prisms continued
Normal
Normal
Virtual
image
Air
Water
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3. Apply Concepts In which
substance is the speed of
light greater, air or water?
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4. Explain If you look into
a body of water, such as a
lake, from a dock or boat,
the water may look shallower
than it actually is. What do
you think is the reason for
this?
Light bends away from the normal
when it moves from the water into
the air. Therefore, the cat sees a virtual
image of the fish that is higher in the
water than the fish actually is.
Air
Water
Light bends toward the normal
when it moves from the air into
the water. Therefore, the fish sees
a virtual image of the cat that is
higher above the water than the
cat actually is.
The images that the cat and the fish see are virtual
images. Light bends away from the normal when it passes
from the water to the air. The cat’s brain interprets the
light as if it traveled in a straight line. Thus, the cat sees a
virtual image of the fish. Similarly, light bends toward the
normal as it passes from the air into the water. Thus, the
fish sees a virtual image of the cat.
How Do Lenses Work?
You may not realize it, but you use the refraction of
light every day. Human eyes, as well as cameras, contact
lenses, eyeglasses, and microscopes, contain parts that
bend light.
When light travels at an angle through a thin, flat
medium, such as a pane of glass, it is refracted twice.
First, it is refracted when it enters the medium. Second,
it is refracted when it leaves the medium. The light ray’s
position as it exits the medium is shifted, but it is still
parallel to the original light ray.
If the medium has a curved surface, the exiting rays
will not be parallel to the original ray. The light will
be traveling in a different direction after it leaves the
medium. Therefore, a curved piece of a transparent
medium, such as glass, can change the direction in which
light travels. Such curved mediums are called lenses.
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Refraction, Lenses, and Prisms continued
TYPES OF LENSES
There are two main kinds of lenses: converging lenses
and diverging lenses. A converging lens, which is thicker
in the middle than at the edges, bends light inward. This
type of lens can create either a virtual image or a real
image. The type of image depends on the distance from
the lens to the object.
A converging lens
focuses light inward.
The light rays focus
to a point called the
focal point of the
lens.
Looking CLoser
5. Identify On the figure,
circle the focal point of the
lens.
A magnifying glass is an example of a converging lens.
If you hold the lens near the object, you see a larger
image of the object through the lens. If you hold the lens
farther from the object, you see a smaller image of the
object.
A converging lens, such as a magnifying glass, can
produce different magnifications. Magnification is any
difference between the size of an image and the size of
the object. A magnification of an object is larger than
the object.
A diverging lens is thinner in the middle than at the
edges. It bends light outward and can create only a
virtual image.
Talk About It
Identify Relationships Look
up the words converge and
diverge in a dictionary. In a
small group, talk about why
different lenses are called
convergent lenses or
divergent lenses.
Looking CLoser
A diverging lens bends light
outward. It can produce only
virtual images.
6. Describe What type of
image can a diverging lens
produce?
MICROSCOPES
A compound light microscope uses multiple lenses to
provide greater magnification than a single magnifying
glass can. The objective lens is close to an object and
forms a large, real image of the object. The ocular lens
in the eyepiece acts like a magnifying glass. It magnifies
the real image from the objective lens. This creates the
large, virtual image you see when you look through the
microscope.
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Refraction, Lenses, and Prisms continued
How Do Human Eyes Work?
Think about a camera that uses film. Light enters the
camera through a large lens. The lens focuses the light
into an image on the film at the back of the camera. Parts
of the human eye work in very similar ways to these
parts of a camera. The figure below shows how the
different parts of the eye affect light.
How the Eye Works
3 The light is refracted again by the lens.
4 The refracted light is
focused onto the back
surface of the inside
of the eye, which is
called the retina.
Muscles around the lens can change its shape
to affect how much the light is refracted.
2 After it passes through the
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7. Explain What can change
how much light refracts
when it passes through the
lens?
cornea, the light passes
through the pupil. The
pupil is the dark hole in
the center of the colored
part of the eye.
5 Special structures on
the retina called rods
and cones detect the
light. Cones detect
colors, but respond
only to bright light.
Rods detect dim light,
but cannot resolve
details well.
1 The cornea is a transparent
membrane that covers the
eye. The cornea refracts light.
6 The signals from the rods and cones
are carried to the brain by the optic
nerve. The brain interprets the signals
to determine what you are looking at.
How Do Prisms Separate White Light Into
Colors?
Remember that white light contains many different
wavelengths, or colors, of visible light. A prism can
separate white light into its component colors. A prism
is a transparent solid that is generally made of glass.
Remember that light waves refract because their speed
changes when they move into a new medium. The
different wavelengths of light travel at different speeds
in a medium, such as the glass in a prism. Therefore, they
bend by different amounts when they pass through the
prism, as shown below.
White light
Red light
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8. Identify Which color of
light bends the most when it
passes through the prism?
Violet light
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DISPERSION
From longest to shortest wavelength, the colors in
the visible spectrum are red, orange, yellow, green, blue,
and violet. Violet light has the shortest wavelength and
travels the most slowly through a medium, such as glass.
Red light has the longest wavelength and travels the most
quickly. Therefore, violet light bends more than red light
when they pass from one medium to another.
Thus, when white light passes from air into a glass
prism, violet bends the most and red bends the least. The
other colors are bent by an amount between violet and
red. When the light exits the prism, the light is separated
into the colors in the visible spectrum. The separation of
light into different colors because of differences in wave
speed is called dispersion.
READING CHECK
9. Identify Which color of
light travels the most slowly?
READING CHECK
10. Define What is
dispersion?
How Do Rainbows Form?
Water droplets in the air can act like prisms. Sunlight
that strikes a water droplet is dispersed into different
colors as it passes from the air into the water. The
refracted light rays strike the back surface of the water
droplet at an angle.
If the angle is small enough, the rays can reflect off
the back of the droplet. Some of the light will then travel
through the droplet. The light is dispersed again as it
passes out of the water droplet back into the air.
Rainbows form through a
combination of dispersion
and reflection.
40º
42º
R
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V
11. Describe What two
effects combine to produce
rainbows?
V
R
When light finally leaves the droplet, violet light
emerges at an angle of 40°. Red light emerges at an angle
of 42°. The other colors of visible light are in between
these angles. We see light from many droplets as arcs of
color, which form a rainbow.
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SECTION VOCABULARY
dispersion in optics, the process of separating
a wave (such as white light) of different
frequencies into its individual component
waves (the different colors)
lens a transparent object that refracts light
waves such that they converge or diverge to
create an image
magnification the increase of an object’s
apparent size by using lenses or mirrors
prism in optics, a system that consists of two or
more plane surfaces of a transparent solid at
an angle with each other
1. Identify Relationships How is dispersion related to refraction?
2. Describe Fill in the blanks in the flowchart below to describe how the human eye
works.
Light enters the eye
Light passes through
The ___________________
through the
the pupil and is refracted
focuses the light on the
________________________,
again by the
back surface of the eye, the
which refracts the light.
________________________.
____________________.
The ________________
__________________ and
___________ carries the
__________________ in the
signals to the brain, which
retina detect the light and
interprets them as images.
convert it to nerve signals.
3. List Give three examples of objects that use converging lenses.
4. Predict Consequences If all wavelengths of light traveled at the same speed in
different mediums, would rainbows occur? Explain your answer.
5. Identify Which type of lens can produce both real and virtual images?
6. Apply Concepts Green light travels more slowly in a medium than yellow light.
Which color of light is bent more when it passes through a prism?
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