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Refraction
When light passes from one medium to another at an angle it changes it’s speed Refraction is the change in direction of a light due to a change in its speed
Refractive power
The ability of the eye to bend (change the direction) light as it passes through the eye.
so that light converges exactly on the retina, which is able to send it to the brain as an information.
The refractive power of the eye is made up of 2 structures: The cornea 2/3 and the lens 1/.
Accomodation
The process by which the eye changes its refractive power to maintain a clear image of an object as it changes its distance.
Refraction Error
Is an error in the focusing (bending) of the light by the eye on the retina.
The problem could be in 1- length of the eye ball 2- Cornea 3- Lens
- Emmetropic +ve refractive error (distant objects)
- Ametropic -ve refractive error (=)
Presbyopia
Describes the loss of the eye's accommodative ability with aging.
After the age of 40, the lens looses its elasticity and becomes gradually hard.
The lens can no longer become globular, The ability to focus on near objects declines.
T.t. 1- reading glasses with corrective lenses
2- Contact lenses 3- Biphocal : eyeglasses with two distinct optic powers 4- Surgery : implantation of accommodative intraocular lenses (IOLs)
Myopia 1
- Close objects are seen clearly, but objects farther away appear blurred.
- Because the convergence occurs Before the retina.
- occurs if the eyeball is too long,, or the cornea has too much curvature. ( Which results in high refractive power for the length of the eye )
Myopia 2
Cataract- increases the refractive power of the lens, making it more myopic.
Patients with extreme degrees of myopia are more susceptible for :- Retinal Detachment Macular Degeneration Primary open angle glaucoma.
