CELL PHYSIOLOGY

DR NILESH KATE

MBBS,MDASSOCIATE PROF

ESIC MEDICAL COLLEGE, GULBARGA. DEPT. OF PHYSIOLOGY

OPTICS.

OBJECTIVES Image forming

mechanism Accommodation. Optical aberration of

the eye. Common defects in

image forming mechanism.

Saturday, February 21, 2015

Image forming mechanism CAMERA

Shutter. Focusing system. Diaphragm. Darkened interior.

EYE Eyelids. Cornea & crystalline lens. Iris. Choroid & pigmented

epithelium of retina.


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.


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 ListingReduced Eye

Simplified data by choosing single principal point & single nodal point.


Schematic eye & reduced eye.


Axes of the Eye. 3 Principal axis

Optical axis. Visual axis. Fixation axis.


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.


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


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.


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.


The Relaxation theory.


Ciliary muscles contract Ciliary ring shortens. Choroid move forwards. Zonules relaxed. Lens capsule relaxed. Lens become more

spherical Increases Diopteric power

of lens

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.


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.


Optical aberration of the Eye. Spherical

Aberration. Spherical lens refracts

peripheral rays more strongly than paraxial rays.


Optical Aberration of the Eye. Chromatic

Aberrations. Index of refraction of

any transparent medium varies with wavelength of the incident of light.


Common defects Emmetropia Ametropia.

Hypermetropia. Myopia. Astigmatism.

Presbiopia.

Emmetropia (optically normal eye)


Ametropia. Hypermetropia. (far sightedness) with accommodation at rest parallel rays of light

coming from infinity focused behind retina.


Ametropia. Myopia(Near sightedness) with accommodation at rest parallel rays of light

coming from infinity focused infront retina.


Optical Correction


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.


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) .


Thank You

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CELL PHYSIOLOGY