Basic optics Flashcards
Refraction - definition
Ability of eye to bend light rays
Refraction - components
Axial length
Refractive media: Cornea, Lens, Aqueous and vitreous humor
Refraction - values
Measured in dioptres
Refractive power of emmetropic eye = 58D
43D from cornea
15D from lens and humor
Refraction testing techniques
Subjective: patient distinguishes effects of different lenses on visibility of letters on Snellen and LogMar charts
Objective: ophthalmoscope, retinoscope
Refraction testing techniques - retinoscopy: use, procedure
Objective refraction
Children under 7 for glasses
In children: with cycloplegia by cyclopentolate drops to prevent accommodation for accurate refraction
Streak of light shone through pupil, seen as red reflex
Streak moved gently and direction of red reflex noted
Place plus and minus lenses in front of pt eye, to calculate short/long sightedness
Accommodation - definition
Ability of eye to focus clearly on an object at any distance
Accommodation - process of focusing on near object
Activate parasympathetics Ciliary muscles contract Suspensory ligaments relax Lens becomes convex Increased refraction of light by lens
Accommodation - what is the far point
Furthest distance at which object can be seen clearly
Accommodation - what is near point
Shortest distance at which object can be seen clearly (maximum accommodation)
Accommodation - what is range of accommodation
Range of distance between far point and near point of accommodation
This range decreases with age
Refractive errors - what is emmetropia, what is ametropia
Normal eye with no refractive error when viewing distant object
Light from distance is focused on retina by non-accommodating
Refractive error when viewing distance object (cannot focus light onto retina)
Refractive errors - hypermetropia: definition, cause, correction
Long-sightedness: see clearly in distance, not near
Short axial length (/low refractive power)
So focal point behind retina - converging rays on retina produce blurred image
Plus lens (convex)
Refractive errors - myopia: definition, cause, correction
Short sightedness: distance vision blurred, near vision normal
Excessive axial length
Excessive refractive power (rare, cataracts)
So focal point in front of retina, divergent rays on retina produce blurred vision
Minus lens (concave)
Refractory errors - astigmatism: definition, cause, correction
Refractive power of eye different at different meridians
Cornea has two axes of curvature
So two foci on retina, produce blurred image
Toric lens (different Power at different meridians)
Refractive error - presbyopia: definition, cause, correction
Loss of accommodation - failure to focus on near object
Lens stiffness + ciliary muscle weakness, due to age
So decreased range of accommodation
Convex lens addition
