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Lenses
Chapter 30
Converging and Diverging Lenses
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Lens – a piece of glass which bends parallel rays so
that they cross and form an image
Converging Lens – a lens which is thicker in the
middle, makes parallel light rays converge
Diverging Lens – a lens which is thinner in the
middle, makes parallel light rays diverge
Principle Axis – line joining the centers of curvature
of a lens’ surface
Focal Point – the point at which a beam of light
converges
Incident parallel beams that are not parallel to the
principle axis focus at points above or below the focal
point
Focal Plane – all possible points from incident
beams
A lens has two focal points and two focal planes
Converging and Diverging Lenses
Image Formation by a Lens
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Magnification occurs when an image is observed
through a larger angle with the use of a lens than
with out the lens, and allows more detail to be seen
A converging lens will only magnify when the object
is between the focal point and the lens
Virtual Image – an image formed through reflection
or refraction that can be seen by an observer but
cannot be projected on a screen because light from
the object does not actually come to a focus
Real Image – an image that is formed by
converging light rays and that can be displayed on a
screen
When a diverging lens is used alone, the image is
always virtual, right-side up, and smaller than the
object
Image Formation by a Lens
Constructing Images Through Ray
Diagrams
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Ray Diagrams – show the principle
rays that can be used to determine that
size and location of an image
One end of the object is always placed
right on the principle axis
3 useful rays in ray diagrams
Ray parallel to the principle axis that
passes through the focal point after
refraction by the lens
A ray through the center of the lens that
does not change direction
A ray through the focal point in front of
the lens that emerges parallel to the
principle axis after refraction by the lens
Constructing Images Through Ray
Diagrams
Image Formation Summarized
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If an object is one focal length away from
a converging lens, the image is virtual,
magnified, and right-side up
If an object is beyond one focal length, a
converging lens produces a real, inverted
image
If the object is close to the focal point,
the image appears far away; if it is far
from the focal point, the image appears
nearer
When an object is viewed through a
diverging lens, the image is virtual,
reduced, and right-side up
Some Common Optical Instruments
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The Camera – consists of a lens and sensitive film
mounted in a lighttight box, the amount of light
which gets to the film is regulated by a shutter
and a diaphragm (which varies the size of the
opening)
The Telescope – a simple telescope uses a lens to
form a real image of a distant object
The Compound Microscope – uses two converging
lenses of short focal length, forming a real and a
virtual image (so we see the object right-side up)
The Projector – An arrangement of converging
lenses is used
Some Common Optical Instruments
The Eye
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The amount of light that enters is
regulated by the iris, the colored part of
the eye that surrounds the opening called
the pupil
Light enters through the transparent
covering called the cornea, passes
through the pupil and lens, and is focused
on a layer of tissue at the back of the
eye—the retina—extremely sensitive to
light
In both the camera and the eye, the
image is upside down, our brain flips the
image right-side up for us
The Eye
Some Defects in Vision
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Farsighted – person’s eyes form
images behind the retina, the
eyeball is too short
Nearsighted – person’s eyes form
images in front of the retina, the
eyeball is too long
Astigmatism – the cornea is
curved more in one direction than
the other, the eye does not form
sharp images
Some Defects in Vision
Some Defects in Lenses
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Aberrations – the distortions in an
image
By combining lenses in certain ways,
aberrations can be minimized
Spherical aberration results when light
passes through the edges of a lens and
focuses at a slightly different place from
light passing through the center of the
lens
Chromatic aberration is the result of the
different speeds of light of various colors
and hence the different refractions they
undergo
Some Defects in Lenses
Diffraction and Interference
Chapter 31
Huygen’s Principle
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Huygen’s Principle – every point on
any wave can be regarded as a new
point source of secondary waves
As a wave front spreads, it appears less
curved
Very far from the original source, the
wave fronts seem to form a plane
You can observe Huygen’s principle in
water waves that are made to pass
through a narrow opening
Huygen’s Principle
Diffraction
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Diffraction – any bending of a wave by means
other than reflection or refraction
When the opening is wide compared with the
wavelength, the spreading effect is small
As the opening becomes narrower, the
spreading of waves becomes more pronounced
Diffraction is not confined to the spreading of
light through narrow slits or other openings,
but occurs to some degree with all shadows
The amount of diffraction depends on the size
of the wavelength compared with the size of
the obstruction that casts the shadow
Diffraction
Interference
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Interference Pattern – at least two
sets of waves create a pattern where
wave effects are increased, decreased,
or neutralized
Constructive Interference – individual
effects add together
Destructive Interference – individual
effects are reduced
Water waves can be produced in shallow
tanks of water known as ripple tanks
Interference
Young’s Interference Experiment
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Thomas Young discovered that when
monochromatic light (light of a single color)
was directed through two closely spaced
pinholes, fringes of brightness and darkness
were produced
The bright fringes of light resulted from light
from both holes arriving crest to crest
The dark areas resulted from light waves
arriving trough to crest
Diffraction Grating – a multitude of closely
spaced parallel slits
Young’s Interference Experiment
Single-Color Interference from Thin
Films
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Interference fringes can be produced by the
reflection of light from two surfaces that are
very close together
You’ll see dark and bright bands when shining
monochromatic light on two plates of glass on
top of each other
The light that has to hit the second surface, has
longer to go to hit your eye, so you see the
interference between the surfaces
Scientists use this in the testing of high
precision lenses
Single-Color Interference from Thin
Films
Iridescence from Thin Films
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Iridescence – the phenomenon whereby
interference of light waves of mixed frequencies
reflected from the top and bottom of thin films
produces a spectrum of color
A thin film, such as a soap bubble, has two
closely spaced surfaces
Light that reflects from one surface, may cancel
out the light that reflects from the second
surface
The different colors in the bubble correspond to
the cancellations of their complementary colors
by different thicknesses of the film
Interference provides the principle method for
measuring the wavelengths of light
Iridescence from Thin Films
Laser Light
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The light from a common lamp is incoherent,
has many phases of vibration
Even if a beam has been filtered to be
monochromatic, it is still incoherent because
the waves are out of phase and interfere with
one another
A beam of light that has the same frequency,
phase, and direction is said to be coherent,
there is no interference of waves within the
beam
Coherent light is produced by a laser (light
amplification by stimulated emission of
radiation)
The laser is not a source of energy, but a
converter of energy
Laser Light
The Hologram
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Hologram – three-dimensional version
of a photograph which contains the
whole message or entire picture in every
portion of its surface
A hologram is produced by the
interference between two laser light
beams on photographic film
One part illuminates the object and is
reflected from the object to the film, the
second part (the reference beam) is
reflected from a mirror to the film
The Hologram
Assignment
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Read Chapter 30 (pg. 463-476)
Do #15-34 (pg. 478-479)
Read Chapter 31 (pg. 480-498)
Do #21-31 (pg. 498)