Transcript Chapter23 - apphysicswarren

Lecture Outline
Chapter 23
College Physics, 7th Edition
Wilson / Buffa / Lou
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23.1 Plane Mirrors
• Mirrors are typically
coated with a
compound of Sn, Al,
or Ag because light
isn’t transmitted
through these
elements.
• The geometry of the
mirror affects the
size, orientation,
and type of image.
23.1 Plane Mirrors
The image formed by a plane mirror is upright,
identical in size to the object, and as far behind
the mirror as the object is in front of it.
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23.1 Plane Mirrors
• Let’s remember…what is:
– d o?
– di?
– Virtual vs. real images?
– Law of Reflection?
We can see all these things on the previous
slide.
Typically scientists are interested in 2 image
features: height and orientation.
23.1 Plane Mirrors
The magnification is given by:
For a plane mirror, M = +1.
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23.1 Plane Mirrors
• For a plane mirror, the image is ALWAYS
– Upright
– Virtual
– Unmagnified
23.2 Spherical Mirrors
A spherical mirror is a section of a sphere. It
may be concave or convex.
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23.2 Spherical Mirrors
A concave mirror (left) focuses incoming
parallel rays at the focal point. A convex mirror
bends incoming parallel rays outward, as
though they came from a focal point behind the
mirror.
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23.2 Spherical Mirrors
• Thus, concave mirrors act as converging
mirrors. (Bring light together)
• Convex mirrors act as diverging mirrors
(light rays move apart)
• Focal Length – Distance from the vertex to
focal point (f).
– f = R/2
• (For mirrors, it’s always half the radius of
curvature)
23.2 Spherical Mirrors
Images made by mirrors can be found by ray
diagramming. There are 3 key rays to draw!
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23.2 Spherical Mirrors
For a concave mirror, the type of image formed
depends on the position of the object.
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23.2 Spherical Mirrors
If the object is at the focal
point, there is no image.
The focal point is a
crossover point between
real and virtual images.
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23.2 Spherical Mirrors
• An object is placed 39 cm in front of a
converging spherical mirror of radius 24
cm.
– A) Use a ray diagram to locate the image
formed by this mirror.
– B) Discuss the characteristics of the image.
23.2 Spherical Mirrors
The spherical-mirror equation is valid for any
object position:
Sign conventions for spherical mirrors are
given on the next slide.
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23.2 Spherical Mirrors
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23.2 Spherical Mirrors
Another way we can calculate the
magnification is given by:
What was the first way again??
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23.2 Spherical Mirrors
• A converging mirror has a radius of
curvature of 30 cm. If an object is placed
– a) 45 cm
– b) 20 cm
– c) 10 cm
from the mirror, where is the imaged formed,
and what are its characteristics?
23.2 Spherical Mirrors
• A candle is 20 cm in front of a diverging mirror that has a
focal length of -15 cm.
• a) Use a ray diagram to determine whether the image
formed is
–
–
–
–
–
–
Real, upright, magnified
Virtual, upright, magnified
Real, upright, reduced
Virtual, upright, reduced
Real, inverted, magnified
Virtual, inverted, reduced
• b) Find the location and characteristics of the image
23.2 Spherical Mirrors
For a convex mirror
the process is
similar, but the
image will always
be virtual.
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23.3 Lenses
A converging lens brings incoming rays
together at the focal point. These are convex
lenses. [Light rays converge at focal point]
Thicker at center
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23.3 Lenses
The rays emerging from a diverging lens appear
to have come from a single focal point. This is a
concave lens. [Thinner at center]
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23.3 Lenses
Both converging and
diverging lenses come
in a variety of shapes.
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23.3 Lenses
• We like to assume the lens are thin.
• If they are not, none of our things work…we
have to do different math that you won’t be
tested on.
• There’s only one big difference between mirrors
and lenses and that is the focal length for lenses
does not equal R/2.
23.3 Lenses
Images formed by lenses can be found just as
mirror images were found. The first two rays:
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23.3 Lenses
Locating and confirming the image:
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23.3 Lenses
• An object is placed 30 cm in front of a thin
converging lens of focal length 20 cm.
– Use a ray diagram to locate the image.
– Discuss the characteristics of the image.
23.3 Lenses
The type of image formed by a converging lens
depends on the position of the object. For a
distant object:
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23.3 Lenses
23.3 Lenses
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23.3 Lenses
The thin-lens equation:
Magnification:
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23.3 Lenses
• A converging lens has a focal length of 12
cm. For an object a) 60 cm, b) 15 cm, and
c) 8.0 cm from the lens, where is the
image formed, and what are the
characteristics?
23.3 Lenses
• A converging lens
forms an image on a
screen. Then the
lower half of the lens
is blocked as shown
in b. As a result,
what happens? Only
the top half of the
image will show;
only the bottom half;
or the entire image
will still be seen?
23.3 Lenses
• An object is 24 cm in front of a diverging
lens that has a focal length of -15 cm.
• a) Use a ray diagram to determine the
characteristics of the image as well as the
location.
23.3 Lenses
A diverging lens always forms a virtual image.
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