FINAL EXAM Flashcards
Optical infinity
In an emmetropic eye, vision is in clear focus when viewing an object that is 20+ feet away. This distance is know as optical infinity. Remember that light rays are essentially parallel from distant objects. The optical media then converge the light rays so that they are in focus on the retina
NEAR POINT
The closest point where we can maintain a clear image with the help of accommodation is called the near point
FAR POINT
. The farthest point that the eye can see clearly is called the far point. The far point in the emmetropic eye is at infinity.
ACCOMMODATION
This readjustment requires an increase in the plus power of the eye (remember, we need more convergence). Amazingly, the eye has the ability to automatically change it’s refracting power in response to a change in distance. This is called accommodation. Accommodation helps to keep things in focus as they move closer
PRESBYOPIA
The accommodative ability decreases with age because of this loss of strength of the ciliary muscle and hardening of the lens. This is referred to as presbyopia. It usually becomes noticeable between 40-45 years of age and gradually worsens from there.
EXPLAIN WHAT HAPPENS DURING ACCOMODATION
This readjustment requires an increase in the plus power of the eye (remember, we need more convergence). Amazingly, the eye has the ability to automatically change it’s refracting power in response to a change in distance. This is called accommodation. Accommodation helps to keep things in focus as they move closer. The closest point where we can maintain a clear image with the help of accommodation is called the near point.
The process of accommodation is complex and there are several theories as to how exactly it happens. But it is known that a change in the shape of the lens allows us to accommodate and to see near objects clearly.
First, the ciliary muscle that surrounds the lens receives a signal to contract. The zonules (fibers that connect the ciliary muscle to the lens) become relaxed. The inherent elasticity of the lens causes it to increase in thickness. This increases its effective power.
During accommodation…
* the ciliary muscle contracts
* the radius of curvature of the central portion of the anterior surface of the lens decreases, increasing its dioptric power
* the thickness of the lens increases
* the anterior surface of the lens moves forward, towards the cornea
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