7.2. VISION, LENSES AND EYE PROPERTIES
Purpose of experiment:
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

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To determine the focal length of a convex (positive or converging) lens
To determine the focal length of a concave (negative or diverging) lens
To calculate the refracting power of both types of lenses
To simulate a short-sighted and long-sighted eye
To determine the position of the blind spot on your left eye retina.
Theoretical topics
 Light refraction.
 Lens parameters: optical center, the main optical axis, the main focus, the focal length and
refracting power.
 Lens formula.
 Image formation by convex and concave lenses.
 Lens aberrations.
 Structure of the retina, formation of images on the retina.
 Eye resolution and visual acuity.
 Correction of lens aberrations.
 Eye accommodation.
 Color vision, night vision, adaptation
 Opthalmoscopy.
Equipment and materials
Optical rail, light source, concave and convex lenses, the eye model, lenses of different
refracting power, tripod, screen, ruler.
Methodology
On the optical rail put light source, object, lenses in holders can slide to different positions
that to create a clear image of the object on the screen (Figure 7.2.1.).
Lens
Light source
Object
Lens
Screen
Obje
ktas
Figure 7.2.1. Measurement set up for optical parameters of lenses
FBML - 7.2. VISION, LENSES AND EYE PROPERTIES
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Lenses produce different types of images, depending on the position of the object relative to
the lens and its focal point.
1. Measurement of focal length of a convex lens
Focal length of the convex lens fc, which can be calculated from the thin lens equation:
1/ fc = 1/p + 1/p1 ,
(7.2.1)
fc = p·p1/(p + p1).
(7.2.2)
or transforming:
m
p1
p
p
p1
n
m
Figure 7.2.2. Measurement of focal length (convex lens)
Measurements should be done three times for both enlarged and reduced sizes of the image to
compare with the size of the object (Fig. 7.2.2.). Note that the focal point of convex lens fc
is positive and situated behind the lens, because the parallel light beams passing through
the convex lens converge making a real focus point F1‘ . Enter the data in Table 1.
Table 1.
p, m
p1, m
f, m
m, m
n, m
f, m
fav., m
2. Measurement of focal length of a concave lens
To measure the a focal length of a concave (diverging) lens fd an additional convex lens of
higher refracting power should be used (Fig. 7.2.3.):
FBML - 7.2. VISION, LENSES AND EYE PROPERTIES
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S
F1
O
F2
F1
K
O1
F2
M
a2
a
1
Figure 7.2.3 Scheme for focal length measurement of a concave lens
And which can be calculated from the equation:
fd = a1·a2/ (a1 - a2).
(7.2.3)
Note that fd is negative – stating that the parallel light beams passing through the concave
lens diverge, and an imaginary focus is placed in front of the lens.
Enter the measured and calculated values into Table 2.
Table 2
a1, m
a2, m
fav., m
D, D
3. Determination of refracting power
Formula 7.2.4 is used to determine the refracting power of both lenses. The refracting power
is inversely proportional to the focal length. The diopter D is the unit for lens refracting
power. 1 diopter is the refracting power of a lens having of focal length f = 1 meter:
D = 1/f .
(7.2.4)
4. The model of the human eye
The eye model is shown in Figure 7.2.4. The model frame is made of organic glass (1)
attached to a wooden table (2). Eye length can be changed by moving the rear wall (3)
(surface of retina) back and forth. The organic glass plate is marked with three positions:
short eyes (hyperopia, farsightedness), normal eyes and long eye (shortsightedness, myopic).
FBML - 7.2. VISION, LENSES AND EYE PROPERTIES
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The lens (4) is made of crystal clear silicone. A
water syringe (5) can change the liquid pressure
inside the lens thus increasing or decreasing the
curvature of lens (at the same focal length or
refracting power).
Images of objects at various distances from
the model are projected onto a white plastic plate
(the retina) (6), and can be adjusted by changing
the shape of the lens. The macula and blind spot
of the retina are marked on the plate. Plate can
be adjusted so that the macula of the eye is
always placed on the axis of the incident light
beam.
4.1.
1
3
6
4
5
2
Figure 7.2.4 The model of human eye
Demonstration of shortsightednesss and farsightedness.
To demonstrate myopia and farsightedness place an object - a transparent plate
engraved with a letter - at a some distance from the eye model.
Then the pressure inside the lens is changed with the syringes to tune the image of the
object in front of the retina (myopic eye) or behind the retina (farsighted eye).
4.2.
Identification of the blind spot on the retina.
Hold a sheet of paper with black cross and black circle 15 cm from it (Fig. 7.2.5.) at a
distance of about half a meter in front of you. Close the right eye and looking with left eye
directly at the cross move the paper toward your eye until the black spot disappears. At that
moment the image of black circle coincides with the blind spot on your retina. Measure the
distance from your eye to the paper sheet. Calculate the angular distance φ from the center of
the macula and blind spot in your eye.
φ = 15/l,
(7.2.5)
Here 15 cm is the distance between the black cross and black spot on the paper sheet, l
– the distance from your eye to the paper sheet when the black circle disappears (distances
measured in cm).
Figure 7.2.5. Determination of blind spot position on the left eye retina
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