Vision 1a


A. Revision of optical terms and concepts from Unit 1 :


A:  Reflection <-> Refraction
B:  Convex <> Concave

C: focal line, focal point, focal length
D: Convergence, Divergence
E:  refractive power = Diopter  (=D) (1 Diopter = 1 meter focal length)
F:         positive or “+ “ lens -> convex
            negative (or virtual or imaginary) or “ – “ lens -> concave




B. Functional Anatomy of the eye:

Sclera: the outer 'skeleton' of the eye. Cornea: the tranparant window into the eye.
Lens: provides for a variable refraction. Ciliary muscle: provides for changes in lens curvature.
Aqueous Humor: fluid in front of the lens that provides for the intra-ocular pressure. Vitreous Humor (or body): provides for a transparant gel mass between lens and retina.
Retina: contains the photoreceptors that are sensitive to the light and the nerve cells that communicate to the optic nerve and the brain. Optic Nerve: nerve bundle (= cranial nerve II) that connects the retina to the occipital cortex in the brain.


C. Accomodation of the eye 1:

A. The eye is not accomodated.

In this situation , when the light travels in parallel rays, from far away, then the focus will fall on the fovea.

B. When the light rays diverge,

because the image is close to the eye (for example when one is reading), then, if nothing changes, the focus will fall behind the fovea, and the picture will be unclear.

C. Accomodation.

To move the focus towards the fovea, the curvature of the lens must become more convex. This will increase the refraction of the lens and the light rays will converge on the fovea.


D. Accomodation of the eye 2:

A. The natural recoil of the lens.

The lens, by itself, when cut of from the suspensory ligaments, is very convex (close to a sphere). It is, in the eye, stretched by the suspensory ligaments and the ciliary muscle into a thinner and lesser convex lens.

B. Non-accomodated.

In this situation, the ciliary muscle is resting and stretches the ligaments, thereby also stretching the lens into a less-convex shape. This is the situation when looking far away.

C. Accomodation.

When the lens has to accomodate (=reading), then the ciliary muscle contracts, the 'hole' in the muscle becomes smaller, the suspensory ligaments move towards the center, and this allows the lens to become more convex.


E. Accomodation of the eye 3: Presbyopia


In some eyes, especially in older people, the elasticity of the eye has decreased and the lens is no longer as convex as before.

A. Maximum accomodation.

In that situation, a maximum accomodation (= contraction of the ciliary muscle) will still not be able to move the focus to the fovea and the image remains blurry. This situation is called presbyopia.

B. Reading lenses.

These presbyopic patients can be helped by giving them reading lenses, which are convex lenses (positive or "+" lenses). They help in breaking the diverging light rays to focus the light rays on to the fovea.




Next: Vision 1b.

free hit counter 2014