PASCO OS-8477A User Manual
Instruction Manual
®
Human Eye Model
OS-8477A
012-13032A
800-772-8700 www.pasco.com
Human Eye Model Table of Contents
Contents
Quick Start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Maintenance and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Adjustable Focus Lens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Experiment Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Experiment 1: Optics of the Human Eye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Experiment 2: Telescope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Experiment 3: Refraction, a Detailed Study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Other Suggested Activities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Teachers’ Notes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Human Eye Model
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Eye Model
Retina Screen
Pupil Aperture
Lenses*
*6 lenses pictured, 12 included
Lens Holder
OS-8468 - Optics Caliper: Expose to light to activate glow-in-the-dark markings.
For use with Human Eye Model (OS-8469) or diffraction pattern measurements.
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OS-8468
OPTICS
CALIPER
(cm)
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OS-8468
OPTICS
CALIPER
(cm)
Optics Caliper
Adjustable Focal
Length Lens
OS-8477A
Included Equipment Part Number
Eye Model OS-8477A
Lens Set:
OS-8476
12 Lenses
1 Retina Screen
1 Pupil Aperture
1 Foam Lens Holder
Optics Caliper OS-8468
Adjustable Focal Length Lens OS-8494
Quick Start
1. Put the retina screen in the NORMAL slot
and the +120 mm lens in the SEPTUM slot.
2. Fill the model with water.
3. Aim the eye at a bright, distant object such
as a window or lamp across the room.
An image is formed on the retina screen.
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Human Eye Model Introduction
Cylindrical Axis
Spherical Lens
Cylindrical Lens
Introduction
The PASCO Human Eye Model consists of a sealed plastic tank shaped roughly like a
horizontal cross section of an eyeball. A permanently mounted, plano-convex, glass
lens on the front of the eye model acts as the cornea. The tank is filled with water,
which models the aqueous and vitreous humors. The crystalline lens of the eye is
modeled by a replaceable lens behind the cornea. The movable screen at the back of
the model represents the retina.
Fixed-focus Lenses
The fixed-focus lenses are equipped with handles, which allow them to be easily
inserted into the water. The handles of the plastic lenses are marked with their focal
lengths in air. Two of the lenses are cylindrical lenses for causing and correcting
astigmatism in the model; these can be identified by notches on their edges that mark
the cylindrical axes. See page 4 for complete lens specifications.
Adjustable Focal Length Lens
The Adjustable Focal Length Lens can be used to model accommodation. See page 5
for instructions on assembling and using the Adjustable Focal Length Lens. The
Adjustable Focal Length Lens is use in Experiment 1, part 2 on page 16.
Lens Positions
The crystalline lens, which is supported in the slot labeled SEPTUM, can be replaced
with different lenses to accommodate, or focus, the eye model at different distances.
(The label refers to the septum, or partition, formed by the lens and other tissues that
separates the aqueous and vitreous humors.) Two other slots behind the cornea,
labeled A and B, can hold additional lenses to simulate changing the power of the
crystalline lens.
A cylindrical lens can be placed in slot A or B to give the eye astigmatism. The pupil
aperture can also be placed into slot A or B to demonstrate the effect of a round or
“cat-shaped” pupil.
Two slots in front of the cornea, labeled 1 and 2, can hold simulated eyeglasses lenses
to correct for near-sightedness, far-sightedness, and astigmatism.
Retina
A circle marked on the retina screen represents the fovea, and a hole in the screen represents the blind spot. The retina screen can be placed in three different positions
(labeled NORMAL, NEAR, and FAR) to simulate a normal, near-sighted, or farsighted eye.
Optics Caliper
The optics caliper can be used to measure images on the retina screen. The tips of the
caliper glow for better visibility in low light.
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Model No. OS-8477A Maintenance and Storage
Retina Screen
Crystalline Lens
Corneal Lens
Demonstration Without Water
The eye model can be used with or without water. With no water, and no changeable
lenses in place (using only the corneal lens), the eye model focuses at optical infinity.
Set the eye model to look out a window to see a large, full-color image of “outside”
on the retina screen.
Maintenance and Storage
The eye model includes two each of 6 different lenses. Put six of them aside as
replacements for lost or damaged lenses; put the other six in the included foam holder
along with the pupil aperture. The lenses are made of polycarbonate plastic, which
has a high index of refraction but scratches easily. Do not wipe or rub the lenses; let
them air dry on a paper towel or in the foam holder. The glass corneal lens can be
cleaned or dried with a soft cloth.
Allow the eye model and its components to dry completely before storing them in a
closed space. When they are dry, place the lenses and lens holder inside the eye model
for storage.
For a set of replacement parts, order PASCO model number OS-8476, which includes
a retina screen, a pupil aperture, two of each lens, and a lens holder.
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Human Eye Model Specifications
Specifications
Model Eye
Dimensions 15 cm × 17 cm × 10 cm
Water Capacity 1 liter
Retina Diameter 7 cm
Movable Lenses
Material Polycarbonate Plastic
Diameter 3 cm
Focal Lengths (In Air)
Spherical Convergent +120 mm
Spherical Convergent +62 mm
Spherical Convergent +400 mm
Spherical Divergent -1000 mm
Cylindrical Convergent +307 mm
Cylindrical Divergent -128 mm
Index of Refraction (Polycarbonate Plastic)
Color Wavelength Index of Refraction
Blue 486 nm 1.593
Yellow 589 nm 1.586
Red 651 nm 1.576
Plano-convex Corneal Lens
Material B270 Glass
Diameter 3 cm
Thickness 4 mm
Radius of Curvature 71 mm
Focal Length (in air) 140 mm
Index of Refraction (Glass)
Color Wavelength Index of Refraction
Blue 486 nm 1.529
Green 546 nm 1.525
Yellow 589 nm 1.523
Red 656 nm 1.520
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Model No. OS-8477A Adjustable Focus Lens
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Syringe
Tubing
Lens Housing
Connector
Figure 1
Membrane
Adjustable Focus Lens
Included Equipment Quantity
1. Adjustable Focal Length Lens
2. Tubing, 5 mm O.D.
3. 10 mL Syringe
2
30 cm
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The Adjustable Focus Lens includes two lenses, a length of plastic tubing, and a
10 mL syringe. Each lens consists of a plastic housing and two flexible membranes.
The syringe is used to fill the lens with a liquid, such as water, and to increase or
decrease the volume of liquid between the membranes. As the liquid volume changes,
the lens curvature and focal length change.
Assembly
Cut a length of tubing about 15 cm long. Attach the piece of plastic tubing to the
syringe and to the connector on the edge of the lens housing (see Figure 1).
Filling the Lens
Follow these steps to fill the lens housing with liquid (such as water or vegetable oil) before using it
with the Human Eye Model. Start with the lens
housing and syringe connected to the plastic tubing
as in Figure 1.
1. Disconnect the syringe from the plastic tubing.
2. Fill the syringe about halfway: Push the piston
all the way into the cylinder. Put the end of the
syringe into the liquid and slowly pull the piston outward so that the liquid is drawn up into
the cylinder. Stop when the liquid level is at
the midpoint of the cylinder.
3. Re-attach the plastic tubing to the syringe.
Hold the syringe vertically so the lens holder hangs down from the end of the
tubing.
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Human Eye Model Adjustable Focus Lens
Figure 2
Retainer ring
Membrane
Lens Housing
4. Do not force the liquid from the syringe into the lens holder. Instead, slowly pull
the piston out so that air from inside the lens holder bubbles up through the liquid
in the cylinder. The liquid should begin moving drop-by-drop into the lens.
5. When the piston is almost to the end of the cylinder, start pushing it back into the
cylinder so that liquid moves from the cylinder into the lens housing.
6. Repeat the process until the lens holder is filled with liquid and there are no air
bubbles in the lens.
7. After the lens holder is filled, make sure that the tubing is also filled with liquid.
Refill the syringe until it is about one-quarter full and reconnect it to the syringe.
Disassembly for Cleaning
Avoid touching the surface of the flexible membranes. If necessary, clean the membrane with a
soft, lint-free cloth moistened with water.
To remove a membrane for cleaning, carefully
remove the retainer ring that holds the membrane
in place and lift the membrane off the lens housing
holding just the edges of the membrane (see
Figure 2).
To reassemble, put the clean membrane on the edge
of the lens housing so that some of the membrane
material extends over the edge. Be sure to center
the membrane on the lens housing. Carefully press
the retainer ring over the membrane and onto the
lens housing.
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Background
Strong Lens Weak Lens
Divergent Lenses
Convergent Lenses
Converging Rays Diverging Rays
How Lenses Form Images
Light rays are bent, or refracted, when they cross an interface between two materials
that have different indices of refraction. The index of refraction of a material is the
ratio of the speed of light in a vacuum to the speed of light in the medium. Light passing through a lens crosses two such interfaces: one where it enters the lens at the front
surface, and another where it leaves the lens at the back surface.
Lenses and Focal Length
The amount by which light is bent is quantified by the lens’s focal length. A strong
lens, which can bend rays so that they intersect at a short distance, is said to have a
short focal length. A weaker lens bends rays less, so that they intersect further away,
and is said to have a long focal length. If the incoming rays are parallel, the distance
at which the outgoing rays intersect is equal to the lens’s focal length.
Human Eye Model
The focal length of a lens is determined by the curvatures of the its front and back surfaces, its index of refraction, and the index of refraction of the material surrounding
the lens. A lens with highly curved surfaces usually has a shorter focal length than
one with flatter surfaces made from the same material. A lens with a high index of
refraction has a shorter focal length than an identically shaped lens with a low index
of refraction. A lens surrounded by air (which has a low index of refraction) has lower
focal length than the same lens immersed in water (which has a high index of refraction).
There are two types of lenses: convergent and divergent. A convergent lens makes
incoming parallel rays converge, or come together. A convergent lens typically has a
convex surface and is thicker at the center than at the edge. The focal length of a convergent lens is positive. A divergent lens makes incoming parallel rays diverge, or
spread apart. A divergent lens typically has a concave surface and is thinner at the
center that at the edge. The focal length of a divergent lens is negative.
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Human Eye Model Background
Real
Image
Virtual
Image
Object Distance Image Distance
Negative Object Distance
The image produced by
the first lens is the object
of the second lens
1
f
---
1
i
---
1
o
---
+=
Focal Points
Images and Image Distance
When an object is placed in front of a lens, the light from the object passing through
the lens forms an image. There are two types of images: real and virtual. A real
image is formed by converging rays at the point where they intersect. A real image
can viewed on a screen placed at that point, and you can see it directly if you place
your eye behind that point. A virtual image is formed by diverging rays at the point
where imaginary lines drawn through the rays intersect. If you allow these diverging
rays to enter your eye, you will see the virtual image located at the point where the
rays appear to be coming from.
The distance from the lens to the image is called the image distance. A real image is
formed behind the lens and has a positive image distance. A virtual image is formed
in front of the lens and has a negative image distance.
Objects and Object Distance
Lenses focus light from an object. The distance from the lens to the object is called
the object distance. In a single-lens system, the object is placed in front of the lens
and the object distance is positive.
In a two-lens system, the object focused on by the second lens is the image (either real
or virtual) formed by the first lens. If this object is in front of the second lens, the
object distance is positive. If the object is behind the lens, the object distance is nega-
tive.
Thin Lens Formula
The focal length of a lens ( f ) is related to object distance (o) and the image distance
(i) by the Thin Lens Formula:
(eq. 1)
If the object is very far from the lens, the object distance is considered to be infinity.
In this case, the rays from the object are parallel, 1/o equals zero, and the image distance equals the focal length. This leads to the definition of the focal point as the
place where a lens focuses incoming parallel rays from a distant object. A lens has
two focal points, one on each side. The distance from the lens to each focal point is
the focal length.
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Model No. OS-8477A Background
M
M
Image Size
Object Size
---------------------------
=
M
M
M
i
o
---
–=
M
Larger, inverted image
Crystalline Lens
Retina
Fovea
Optic
Nerve
Corneal
Lens
Aqueous
Humor
Pupil
Vitreous Humor
Iris
Blind Spot
Horizontal Cross Section of the Human Eye
Magnification
The size of an image can be different from the size of the object. The relative magnification, , of the image is defined by:
(eq. 2)
If is greater than 1, the image is larger than the object; if is less than 1, the
image is smaller.
The magnification, M, which can be positive or negative, represents both the size
and orientation of the image. It can be defined in terms of the image and object distances:
(eq. 3)
If M is positive, the image is upright, or in the same orientation as the object. If M is
negative, the image is inverted, or in the opposite orientation to the object. If the
object is right-side-up, then the inverted image appears upside-down.
In the pictured example (right), the image is larger than the object and inverted, which
means that is less than 1 and M is negative.
Anatomy of the Eye
The human eye achieves vision by
forming an image that stimulates nerve
endings, creating the sensation of sight.
Like a camera, the eye consists of an
aperture and lens system at the front,
and a light-sensitive surface at the back.
Light enters the eye through the aperture-lens system, and is focused on the
back wall.
The lens system consists of two lenses:
the corneal lens on the front surface of
the eye, and the crystalline lens inside
the eye. The space between the lenses is
filled with a transparent fluid called the
aqueous humor. Also between the
lenses is the iris, an opaque, colored
membrane. At the center of the iris is
the pupil, a muscle-controlled, variable-diameter hole, or aperture, which
controls the amount of light that enters
the eye.
The interior of the eye behind the crystalline lens is filled with a colorless, transparent
material called the vitreous humor.
On the back wall of the eye is the retina, a membrane containing light-sensitive nerve
cells known as rods and cones. Rods are very sensitive to low light levels, but provide us only with low-resolution, black-and-white vision. Cones allow us to see in
color at higher resolution, but they require higher light levels. The fovea, a small area
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Human Eye Model Background
Object
Lens
System
Retina
Image
Normal Vision
Corrected Myopia
Myopia
Normal Vision
Corrected Hypermetropia
Hypermetropia
near the center of the retina, contains only cones and is responsible for the most acute
vision. Signals from the rods and cones are carried by nerve fibers to the optic nerve,
which leads to the brain. The optic nerve connects to the back of the eye; there are no
light-sensitive cells at the point where it attaches, resulting in a blind spot.
Optics of the Eye
The corneal lens and crystalline lens
together act like a single, convergent
lens. Light entering the eye from an
object passes through this lens system
and forms an inverted, real image on
the retina.
The eye focuses on objects at varying
distances by accommodation, or the use of muscles to change the curvature, and thus
the focal length, of the crystalline lens.
In its most relaxed state, the crystalline lens has a long focal length, and the eye can
focus the image of a distant object on the retina. The farthest distance at which the eye
can accommodate is called the far point for distinct vision. For a normal eye, the far
point is infinity.
When muscles in the eye contract and squeeze the lens, the center of the lens bulges,
causing the focal length to shorten, and allowing the eye to focus on closer objects.
The nearest distance at which they eye can accommodate is called the near point for
distinct vision. The near point of a normal eye is about 25 cm.
Visual Defects and their Correction
A normal eye can focus by accommodation on any object more than about 25 cm
away. In cases where an eye cannot focus on an object, the image is formed either
behind or in front of the retina, resulting in blurred vision. This can be caused by the
eye being too short or too long.
Near-sightedness (Myopia)
A person affected by myopia has an eye ball that is too long, making the distance
from the lens system to the retina too large. This causes the image of distant objects to
be formed in front of the retina. The far point of a myopic eye is less than infinity.
A myopic eye can naturally focus divergent rays from a near object on the retina, but
not parallel (or nearly parallel) rays from a distant object. Eyeglasses that correct
myopia have a divergent lens, which forms a virtual image of the distant object closer
to the eye.
Far-sightedness (Hypermetropia)
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A person affected by hypermetropia has an eye ball that is too short, making the distance from the lens system to the retina too small. This causes the image of near
objects to be formed behind the retina. The near point of a hypermetropic eye is
greater than normal.
A hypermetropic eye can naturally focus parallel (or nearly parallel) rays from a distant object on the retina, but not highly divergent rays from a near object. Hypermetropia can be corrected using eyeglasses that have a convergent lens, which reduces
the divergence of incoming rays.
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Model No. OS-8477A Background
Vertical- and Horizontal-line Objects
Astigmatic Lens System
of the Eye
Horizontal-line image
formed at the retina
Vertical-line image formed
behind the retina
Corrective Cylindrical Lens
Both images formed
at the retina
Perceived
Image
Perceived
Image
ObjectImage Formed by
Magnifying Glass
Magnifying
Glass
Eyes Lens
System
Image Formed
on Retina
A form of hypermetropia called presbyopia (old-sightedness) is not caused by the
shape of the eye, but by a change in the crystalline lens: over time, the lens becomes
more rigid, making it less able to accommodate to short object distances.
Astigmatism
Astigmatism is a defect caused
by a lack of rotational symmetry
in the lens system. The lens is not
spherical (as in a normal lens) but
has two unequal focal lengths.
This makes the eye able to focus
sharply only on lines of a certain
orientation, and all others look
blurred. Astigmatism is corrected
with an cylindrical eyeglasses
lens that has no curvature in one
plane and the correct amount of
curvature in the other plane. The
combination of the eye’s defective lens and the cylindrical corrective lens is equivalent to a
single symmetric spherical lens.
The alignment of the corrective
lens is critical; rotating the eyeglasses lens with respect to the
eye will ruin the effect.
In the example illustrated (right),
the vertical- and horizontal-line
objects are at the same distance,
but the eye’s lens system forms an
image of the vertical line at a
greater distance than the image of
the horizontal line. If the horizontal-line image is formed exactly at
the retina then it appears in focus,
but the vertical-line image is
formed behind the retina and
appears blurred. The corrective
lens causes the vertical- and horizontal-line images to be formed at the same distance.
Optical Instruments
Optical instruments enhance vision by forming an image that is a different size or at a
different position from the object.
A magnifying glass is a single
convergent lens used to view
near objects. It creates a real,
upright image that is larger and
farther away. This makes the
object appear larger, and it allows
the eye to focus on an object that
would normally be closer than
the eye’s near point. The object
distance must be less than the
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