2. Objectives :
Eye and its components.
Axis and visual angles of eye .
Describe the properties of light .
Differentiate between convex and concave lenses.
Describe Accommodation.
Describe how the eye focuses light.
Differentiate between presbyopia , Myopia , hyperopia
and astigmatism.
3. Eye as a camera :
Eyelids – shuttter .
Cornea – focusing system.
Lens – focusing system.
Iris – diaphragm .
Choroid – dark chamber.
Retina – light sensitive film.
4. Cornea :
Cornea is a plus lens of +42D
Radius of curvature of ant.surface =
7.8mm
post.surface = 6.8mm
Diameter about 11.5mm and thickness of
0.5 – 0.6mm in centre and 0.6 – 0.8mm
periphery.
An average horizontal diameter = 11.7
mm
vertical diameter = 11mm.
5. Contd...
Reasons of refraction :
curvature
Significant differences in refractive
index .
6. Crystalline lens :
Lens accounts for about one-third of refraction of
eye.
accomodation
provides a mechanism of focusing at different diatances.
Radius of curvature
ant.surface = 10mm
post.surface = 6mm
Refractive index
nucleus- 1.41
pole – 1.385
cortex – 1.38
Total power – 15 -18 D.
7. Anterior chamber :
Is a cavity between cornea and iris .
Filled with aqueous humour .
Refractive index of aqueous humour is
1.336.
depth of aqueous chamber is 2.5 -
4.00mm
change in anterior chamber depth ,
change the total power.
8. Iris and pupil :
Regulates the amount of Light entering
the eye.
at 2.4mm Pupil size, best retinal image
obtained, as abberation and diffraction
are balanced.
pupil size varies 2-4mm in diameter in
bright light
4-8mm in diameter in dim light .
9. Vitreous humour :
Is a transparent ,colorless ,gelatinous
mass that fills the space in between lens
and the retina .
is 8-9mm in diameter.
Refractive index of vitreous humour is
1.337.
10. Retina :
Maximum resolving power is at fovea
light sensitive film.
Concave spherical surface with radius of 12mm.
22mm in diameter .
contains about 7 million cones and 75- 150 million rods.
Advantages of curvature of retina is that
Curved images formed by the optical system Is brought in the right order.
A much wider field of view Is covered by the steeply curved retina .
11. Axis and visual angles of eye :
Optical axis – the line passing through
the centre of cornea and centre of lens
and meets the retina on the nasal side of
the fovea.
Visual axis – is the line joining the
fixation point , nodal point and the fovea.
12. Axis and visual angles of eye :
Fixation axis – is the straight line that
joins centre of rotation of eyeball with
fixation point .
13. Visual angles :
Angle alpha – angle formed between
visual axis and optical axis at nodal point.
Angle gamma – angle between optic
axis and fixation axis .
Angle kappa – angle formed between
pupillary line and visual axis .
14. Properties of light
Electromagnetic radiation
Visible light
Wavelength of 400- 700nm
Violet- short wavelength , highest energy .
Red- long wavelength , lowest energy.
15. Properties of light
Reflection
Light blocked by non-
transparent object and bounce
off a surface .
Refraction
Bending of light as it passes from
another with a different density.
16. Focusing light on the retina :
Light is bent 3 times
Enters the cornea
Enters the lens
Leaving the lens
Only can adjust the amount of refraction
through lens.
17. Convex And concave lenses :
Convex lenses
Light converges as it passes through.
Converge to single point = focal point
A thick lens causes more refraction and therefore
shorter focal point .
A thin lenses causes less refraction and therefore
longer focal point..
Results in image being upside down and
reversed at focal point.
18. Convex and concave lenses
Concave lenses
Diverge ( spreads out the light rays that
have been refracted through it )Light .
20. Emmetropic eye
Distance
Light coming to eye as parallel rays
Dont need to bend light as much
Ciliary muscles are relaxed
21. Emmetropic eye
Near vision
Light coming in as a divergent rays .
Need to thicken the lens .
ciliary muscles contracted.
22. Focusing for close vision
Accommodation Of lenses
its the ability of eye to change its focus from distant to near objects ,
this process is achieved by the lens changing its shape .
Ciliary muscles contract so lens can thicken.
Far point of vision = Farthest point till where we can see.
That distance in which no accommodation Is needed ( 6m Or 20ft ).
near point of vision= the nearest point
Closest point on which we can focus clearly ( lens cant get any fatter )
23. Contd...
Constriction of pupils
Prevents the most divergent light rays from entering
the eye .
convergence of eyeballs
Keeps object focus on fovea .
getting cross eyed.
24. Myopia :
Near sightedness
parallel rays of light are focused in front
of retina when accommodation is at rest .
occurs when eyeball is too long or
cornea becomes too steeply curved .
Can correct through concave lenses.
25. Hyperopia:
Long sightedness.
parallel rays of light coming from infinity
are focused behind the retina with
accommodation being at rest.
Occur if eye is too short
can correct through convex lens .
