# Lenses by yurtgc548

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By Micah Massari and Dean Yoder
Converging and Diverging
Lenses
• A lens is a piece of glass that can bend parallel rays of light
so they cross or appear to have crossed.
• Lenses can cause light to converge to a single point or
diverge from a single point.
• A converging lens is thicker in the middle than on the
outsides and a diverging lens is thinner in the middle then
on the outside.
• Principal axis of a lens is the line joining the centers of
curvatures of its surface.
• Focal point is the point at which light converges when
beamed parallel to the principal axis. A focal plane is made
up of beams that focus above or below the focal plane.
• Focal length the distance between the center of the lens
and its focal point.
Image Formation by a Lens
• Far away objects are seen through a small angle of view
while closer objects are seen through a larger angle of
view. The wider angle allows you to see more detail.
• Converging lenses help magnify or let you look at
something through a wider angle.
• Magnifying glasses can only magnify an object within its
focal length.
• When an object is farther away than the focal point of a
converging lens light does converge and it can be focused
on a screen as a real image.
• Diverging lenses are used as view finders for cameras the
virtual image you see is right side up and smaller than the
object.
Constructing Images Through
Ray Diagrams
•   Ray diagrams are used to determine the location and
size of an image. The image can be located by following
light ray paths, there are three:
1.   The first ray starts parallel to the principal axis and then is refracted
to the focal point.
2.   The next path is through the center of the lens and continue in a
strait line.
3.   The third path is when a ray passes through the focal point of the
lens and emerges parallel to the principal axis.
–   Only two of these rays are needed to find locate an
image.
–   Ray diagrams can also be used for diverging lenses.
Image Formation Summarized
• When an object is within one focal length it is a simple
magnifying glass that produces a magnified , right side up,
virtual image. When an object is beyond one focal length it
produces a real inverted image.
• When an object is viewed with a diverging lens the image
is virtual, reduced, and right side up.
Common Optical Instruments
• The camera- a lens and sensitive film are mounted in a light
tight box. The lens forms a real inverted image on the film.
• The telescope- a lens forms a real image of a distant object
the image is projected into space to be viewed by another
magnifying like lens.
• Binoculars- are like two telescopes side by side except they
have prisms inside to flip images right side up. They are
less bright than a normal telescope with two lenses because
the light has to pass through all of the prisms making them
less bright.
Common Optical Instruments
• Compound microscope- uses two converging lenses with
short focal length. The objective lens is the first lens; it
produces a real image of a close up object. The eye piece is
the second lens and it forms a virtual image that is even
more enlarged.
• Projector- a concave mirror reflects light from an intense
light source onto a pair of condenser lenses; these direct
the light through the slide to a projection lens. The
projection lens is mounted so it can be moved back and
forth for focusing.
The Eye
• Iris- regulates the amount of light that can enter your eye
and it also surrounds the pupil.Light enters through the
cornea then it passes through the pupil and lens and is
focused on the retina.
• The fovea is the most sensitive part of the retina and allows
the view of greater detail.
• You see images upside down but your brain has learned to
flip it.
• To focus an image your eyes accommodate by thickening or
thinning your lens by using ciliary muscles that surround the
lens.
• Vision is sharpest in the eye when the pupil is the smallest.
Some Defects in Vision and in
Lenses
• People with normal vision can see objects from infinity
• Farsighted- people with this problem form images behind
their retina so they must hold objects more than 25 cm
away to see it clearly. To fix this the person may wear
glasses with converging lenses.
• Nearsighted- the person can see objects nearby very
clearly but far away objects are out of focus. This is
because they form images in front of the retina. This
problem is fixed by wearing glasses with diverging lenses.
Some Defects in Vision and in
Lenses
• Astigmatism- this is caused when one part of the cornea
is bent more than the other. For this cylindrical glasses are
prescribed.
• No lens can give a perfect image combining lenses can
reduce aberrations though making clearer images.
• Spherical aberration is caused when light rays that pass
through a lens focus at different points. This is corrected
by adding a diaphragm to the outside or combining other
lenses.
• Chromatic aberration is caused by the different speeds of
different colors of light which causes different refractions.
This is fixed by using Achromatic lenses which are lenses
combined to correct this effect.

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