OPTICS

1. DR NILESH KATE
MBBS,MD
ASSOCIATE PROF
ESIC MEDICAL COLLEGE, GULBARGA.
DEPT. OF PHYSIOLOGY
OPTICS.

2. OBJECTIVES
 Image forming
mechanism
 Accommodation.
 Optical aberration of
the eye.
 Common defects in
image forming
mechanism.
Saturday, February 21, 2015

3. Image forming mechanism
CAMERA
 Shutter.
 Focusing system.
 Diaphragm.
 Darkened interior.
EYE
 Eyelids.
 Cornea & crystalline lens.
 Iris.
 Choroid & pigmented
epithelium of retina.
Saturday, February 21, 2015

4. Optics of Eye.
 Focusing system of eye with refracting
structures. (Refractive Indices)
 Cornea.(1.37)
 Aqueous humour.(1.33)
 Crystalline lens. (1.42)
 Vitreous humour. (1.33)
 So total Diopteric power of eye +60D
 +44D by cornea
 +16D by crystalline lens.
Saturday, February 21, 2015

5. Cardinal points of eye.
By Listing & Gauss
Schematic Eye.
 For homocentric lens
system of eye there are 3
cardinal points
 Principal foci (2)
 Principal points (2)
 Nodal points (2)
By Listing
Reduced Eye
 Simplified data by
choosing single principal
point & single nodal point.
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6. Schematic eye & reduced eye.
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7. Axes of the Eye.
 3 Principal axis
 Optical axis.
 Visual axis.
 Fixation axis.
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8. Accommodation.
 Def – Mechanism to focus the diverging rays
coming from near objects on the retina to see
it clearly.
 There ids increase in power of the crystalline
lens.
Saturday, February 21, 2015

9. Accommodation
 Near point / Punctum proximum (P)– nearest
point at which objects can be seen clearly.
 Far point / Punctum remotum (R) – distant point
at which objects can be seen clearly.
 Range of accommodation – far point – near point.
 Amplitude of accommodation -- Difference in
dioptic power P-R
Saturday, February 21, 2015

10. Mechanism of Accommodation.
 Fish – retract the lens
 Snakes & frogs – move the lens forwards
 Horses -- move the head & tilt the retina
 Human Being – change the shape of the lens.
Saturday, February 21, 2015

11. The Relaxation theory.
 Thomas Young &
Helmholtz (1885)
 At rest lens
compressed in
capsule by tension
of zonules d/t
relaxation of fibres
of ciliary muscle.
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12. The Relaxation theory.
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 Ciliary muscles contract
 Ciliary ring shortens.
 Choroid move forwards.
 Zonules relaxed.
 Lens capsule relaxed.
 Lens become more
spherical
 Increases Diopteric power
of lens

13. Optical changes.
 Slackening of the zonules.
 Changes in curvature of lens surface.
 Anterior pole move forwards.
 Axial thickness increases.
 Tension of lens capsule decreases.
 Lens sinks down.
 Lens substances change internally.
 Pupillary constriction & convergence of eyes.
 Choroid & Ora serrata move forwards.
Saturday, February 21, 2015

14. Optical aberration of the eye.
 Def – lapses from
perfections.
 Diffraction of light.
 Caused by edge of an
aperture or rim of the
lens.
 Actual pattern of
diffracted image –
alternate dark & bright
concentric rings.
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15. Optical aberration of the Eye.
 Spherical
Aberration.
 Spherical lens refracts
peripheral rays more
strongly than paraxial
rays.
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16. Optical Aberration of the Eye.
 Chromatic
Aberrations.
 Index of refraction of
any transparent
medium varies with
wavelength of the
incident of light.
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17. Common defects
 Emmetropia
 Ametropia.
 Hypermetropia.
 Myopia.
 Astigmatism.
 Presbiopia.
 Emmetropia (optically
normal eye)
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18. Ametropia.
 Hypermetropia. (far sightedness)
 with accommodation at rest parallel rays of light
coming from infinity focused behind retina.
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19. Ametropia.
 Myopia(Near sightedness)
 with accommodation at rest parallel rays of light
coming from infinity focused infront retina.
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20. Optical Correction
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21. Astigmatism.
 Refraction varies in
different meridia.
 Regular.
 Refractive power
changes uniformly
from one meridia to
other.
 Irregular
 Refractive power
changes irregularly
from one meridia to
other.
Saturday, February 21, 2015

22. Presbiopia.
 Eyesight of old age.
 Due to physiological
insufficiency of
accommodation.
 Near vision fails
initially.
 After 40 , near point
recedes beyond the
normal reading range
(25 cm) .
Saturday, February 21, 2015

23. Thank
You