Download Lenses form images by refracting light.

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KEY CONCEPT
Lenses form images by
refracting light.
BEFORE, you learned
NOW, you will learn
• Waves can refract when
they move from one
medium to another
• Refraction changes the
direction of a wave
• How a material medium can
refract light
• How lenses control refraction
• How lenses produce images
VOCABULARY
EXPLORE Refraction
lens p. 121
focal length p. 123
How does material bend light?
PROCEDURE
1
Place the pencil in the cup, as shown in
the photograph. Look at the cup from the
side so that you see part of the pencil
through the cup.
MATERIALS
•
•
•
•
clear plastic cup
pencil
water
mineral oil
2 Fill the cup one third full with water and
repeat your observations.
3 Gently add oil until the cup is two-thirds full. After
the oil settles into a separate layer, observe.
WHAT DO YOU THINK?
• How did the appearance of the pencil change when
you added the water and the oil?
• What might explain these changes?
A medium can refract light.
When sunlight strikes a window, some of the light rays reflect off the
surface of the glass. Other rays continue through the glass, but their
direction is slightly changed. This slight change in direction is called
refraction. Refraction occurs when a wave strikes a new medium—such
as the window—at an angle other than 90˚ and keeps going forward in
a slightly different direction.
Refraction occurs because one side of the wave reaches the new
medium slightly before the other side does. That side changes speed,
while the other continues at its previous speed, causing the wave to turn.
Check Your Reading
How does the motion of a light wave change when it refracts?
Chapter 4: Light and Optics 119
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COMBINATION NOTES
Sketch the ways light is
refracted when it moves
into a denser medium and
into a thinner medium.
Refraction of Light
Recall that waves travel at different speeds in different mediums.
The direction in which a light wave turns depends on whether the
new medium slows the wave down or allows it to travel faster. Like
reflection, refraction is described in terms of an imaginary line—called
the normal—that is perpendicular to the new surface. If the medium
slows the wave, the wave will turn toward the normal. If the new
medium lets the wave speed up, the wave will turn away from the
normal. The wave in the diagram below turns toward the normal
as it slows down in the new medium.
air
(thin)
1
normal
light
wave
reading tip
A dense medium has more
mass in a given volume than
a thin medium.
light
air
(thin)
glass
(dense)
2
1
Waves moving at an
angle into a denser
medium turn toward
the normal.
2 Waves moving at an
angle into a thinner
medium turn away
from the normal.
Light from the Sun travels toward Earth through the near vacuum
of outer space. Sunlight refracts when it reaches the new medium of
Earth’s upper atmosphere. Earth’s upper atmosphere is relatively thin
and refracts light only slightly. Denser materials, such as water and
glass, refract light more.
By measuring the speed of light in different materials and comparing this speed to the speed of light in a vacuum, scientists have been
able to determine exactly how different materials refract light. This
knowledge has led to the ability to predict and control refraction,
which is the basis of much optical technology.
water
droplet
color
spectrum
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Light passing through
a droplet of water is
refracted twice, forming
a color spectrum.
120 Unit: Waves, Sound, and Light
Refraction and Rainbows
You’ve seen rainbows in the sky after a rainstorm or
hovering in the spray of a sprinkler. Rainbows are
caused by refraction and reflection of light through
spherical water drops, which act as prisms. Just as a
prism separates the colors of white light, producing
the color spectrum, each water drop separates the
wavelengths of sunlight to produce a spectrum. You
can see the effect in the diagram on the left.
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Shape determines how lenses form images.
When you look at yourself in a flat mirror, you see your image clearly,
without distortions. Similarly, when you look through a plain glass
window, you can see what is on the other side clearly. Just as curved
mirrors distort images, certain transparent mediums called lenses alter
what you see through them. A lens is a clear optical tool that refracts
light. Different lenses refract light in different ways and form images
useful for a variety of purposes.
reading tip
Distort means to change
the shape of something
by twisting or moving the
parts around.
Convex and Concave Lenses
Like mirrors, lenses can be convex or concave. A convex lens is curved
outward; a concave lens is curved inward. A lens typically has two
sides that are curved, as shown in the illustration below.
Convex Lens
Concave Lens
focal
point
principal axis
A convex lens causes parallel light
rays to meet at a focal point.
A concave lens causes parallel light
rays to spread out.
Parallel light rays passing through a convex lens are refracted
inward. They meet at a focal point on the other side of the lens. The
rays are actually refracted twice—once upon entering the lens and
once upon leaving it. This is because both times they are entering a
new medium at an angle other than 90 degrees. Rays closest to the
edges of the lens are refracted most. Rays passing through the center
of the lens—along the principal axis, which connects the centers of
the two curved surfaces—are not refracted at all. They pass through
to the same focal point as all rays parallel to them.
Convex
reminder
The focal point is the point
at which parallel light rays
meet after being reflected
or refracted.
Parallel light rays that pass through a concave lens are
refracted outward. As with a convex lens, the rays are refracted twice.
Rays closest to the edges of the lens are refracted most; rays at the very
center of the lens pass straight through without being deflected.
Because they are refracted away from each other, parallel light rays
passing through a concave lens do not meet.
Concave
Check Your Reading
Compare what happens to parallel light rays striking a concave
mirror with those striking a concave lens.
Chapter 4: Light and Optics 121
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How a Convex Lens Forms an Image
A convex lens forms an image by
refracting light rays. Light rays
reflected from an object are refracted
when they enter the lens and again
when they leave the lens. They meet
to form the image.
ray A
focal
point
focal lengths
1
ray B
2
Light rays reflect off the penguin in all
directions and many enter the lens. Here a
single ray (A) from the top of the penguin
enters the lens and is refracted downward.
point
C
Another light ray (B) from the top of the
penguin passes through the lens at the bottom
and meets the first ray at point C. All of the
rays from the top of the penguin passing
through the lens meet at this point.
point
D
3
All of the light rays from the bottom of the
penguin meet at a different point (D).
Light rays from all parts of the penguin meet
at corresponding points on the image.
Where do light rays reflected from the middle of the penguin meet?
D
C
122 Unit: Waves, Sound, and Light
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Images Formed by Lenses
When light rays from an object pass through a lens, an image of
the object is formed. The type of image depends on the lens and, for
convex lenses, on the distance between the lens and the object.
SIMULATION
CLASSZONE.COM
Work with convex and
concave lenses to form
images.
Notice the distance between the penguin and the lens in the
illustration on page 122. The distance is measured in terms of a
focal length, which is the distance from the center of the lens to
the lens’s focal point. The penguin is more than two focal lengths
from the camera lens, which means the image formed is upside
down and smaller.
If the penguin were between one and two focal lengths away from
a convex lens, the image formed would be upside down and larger.
Overhead projectors form this type of image, which is then turned
right side up by a mirror and projected onto a screen for viewing.
Finally, if an object is less than one focal length from a
convex lens, it will appear right side up and larger. In order to
enlarge an object so that you can see details, you hold a magnifying lens close to the object. In the photograph, you see a
face enlarged by a magnifying lens. The boy’s face is less than
one focal length from the lens.
If you look at an object through a concave lens, you’ll see
an image of the object that is right side up and smaller than
the object normally appears. In the case of concave lenses, the
distance between the object and the lens does not make a difference in the type of image that is formed. In the next section
you’ll see how the characteristics of the images formed by different
lenses play a role in complex optical tools.
When will an image formed by a convex lens be upside down?
KEY CONCEPTS
CRITICAL THINKING
1. What quality of a material
affects how much it refracts
light?
4. Infer You look through a lens
and see an image of a building
upside down. What type of
lens are you looking through?
2. How does the curve in a lens
cause it to refract light differently from a flat piece of glass?
3. How does a camera lens form
an image?
5. Make a Model Draw the
path of a light ray moving at
an angle from air into water.
Write a caption to explain
the process.
CHALLENGE
6. Study the diagram on the
opposite page. Describe the
light rays that would pass
through the labeled focal
point. Where are they coming
from, and how are they related
to each other?
Chapter 4: Light and Optics 123
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