Concepts Flashcards
(194 cards)
IMAGE DISPLACEMENT
Image displacement — produced by the total prismatic power acting in the reading position (the total prismatic power of the lens plus the bifocal segment).
Critical angle
Only occurs when light passes from a higher index to a lower index medium.
Light ray passing through mediums (lower/higher) index of refraction to (higher/lower) index of refraction
When a light ray passes from a medium with a lower refractive index (n) to a medium with a higher refractive index (n1), it is bent toward the normal.
When passing from a higher refractive index (n1) to a lower refractive index (n), it is bent away from the normal.
Image jump
Image jump — produced by sudden introduction of prismatic power at the top of the bifocal segment. The object which the eye sees in the inferior field when looking straight ahead suddenly jumps upward when the eye turns down to look at it
If the optical center of the segment is at the top of the segment, there is no image jump:
Diopters
diopters — the reciprocal of the distance, in meters, to the point where the rays would intersect if extended in either direction.
Chromatic aberration
Simple plus lenses likewise bend blue rays more than red rays, leading to the optical aberration known as chromatic aberration. The blue rays come to focus closer to the lens than the red rays.
Light with shorter wavelengths are SLOWED UP more in media (compared to longer wavelengths)
Principle behind duochrome test.
Which color light is bent more due to refraction?
Blue = bent more due to refraction.
Longer wavelength, lower energy, refracted lens.
Duochrome test
RAM-GAP
Red side better, refraction = too hyperopic (ADD MINUS)
Green side better, refraction = too myopic (ADD PLUS)
Chromatic aberration occurs strongly in the human eye, with almost 3.00 D difference in the focus of the far ends of the visible spectrum (1.50 D is usually stated in textbooks).
basis of duochrome test (aka bichrome test).
Sphere is adjusted until the black letters on the red and green halves of the test chart are equally clear, indicating that the red rays are focused as far behind the retina as the green rays are focused in front. Yellow light, midway between the red and green, will then be in perfect focus on the retina, the optimum focus when viewing with white light.
The red and green filters usually used create a chromatic spherical difference of only 0.50 D, requiring visual acuity of 20/30 or better to distinguish a blur difference. Balance with the red-green test should always be approached from the fogged direction (red clearer) to minimize accommodation.
Vergence
vergence = measure of the curvature of the wavefront. (The more curved the wavefront, the greater the vergence.)
Curvature = reciprocal of the radius
Lenses add vergence to light.
power of the lens (diopters) = amount of vergence added to the light
Negative vergence: rays diverging
Positive vergence: rays converging
Which color light is bent more due to diffraction?
Red is DIFFRACTED MORE
Focal point
all parallel rays cross at the focal point (part of definition). Focal point = reciprocal of the lens
Circle of least confusion of a lens
take the spherical equivalent of the prescription. Then take the reciprocal of the diopter = distance of the circle of least confusion.
How should glass prisms be held with the line of sight?
Easy way to think about it: hold it the usual way; plastic prisms (usual ones) are parallel to the FACE
Prentice position: Glass prisms should be held with the back surface PERPENDICULAR to the line of sight (parallel to the eye)
If you don’t hold it this way, then you measure more deviation then what the patient actually has (therefore if you use these incorrect measurements, you will over-correct)
Dk/L
best at describing how much oxygen can pass through a contact lens and reach the cornea. Good way of comparing different brands of contact lenses.
Dk = oxygen transmisibility of a material.
L = central thickness of the lens
How should plastics prisms be held with the line of sight?
Easy way to think about it: hold it the usual way; parallel to the face
plastics prisms should be held with the rear surface in the frontal plane. For near fixation objects, the rear surface should be angled in slightly so as to be perpendicular to the direction of the fixation object.
reduced schematic eye numbers
distance between anterior corneal surface and nodal point of the eye = 5.5 mm
distance between nodal point of eye and retina =17 mm
total axial length of the eye = 22.5 mm
total power of the eye = 60 D
index of refraction of schematic eye = 1.33
what do you use the reduced schematic eye for?
solving problems involving:
1) calculating image size that an object projects on the retina
2) calculating the size of the projected blind spot when given the size of the optic disc.
silicone oil replacing vitreous in pseudophakic and aphakic eye
Pseudo/phakic eyes: hyperopic shift of 3-5 D (b/c oil presses up and molds to lens as a CONCAVE shape and acts as a negative powered lens) –> hyperopic shift
Aphakic eye (silicone oil has a CONVEX shape –> + lens –> myopic shift)
constructive/destructive interferance
electromagnetic waves combine to produce wave of greater/lesser intensity
temporal coherence
one part of a beam of light is able to interfere with another part of the same beam of light
spatial coherence
when 2 parts of the SAME wave can produce interference. This is how diffraction gradings are produced.
Knapp’s law
Spectacles in anterior focal plane (~15.7mm) induce no aniseikonia
If anisometropia (difference in refraction) is due to an axial length difference, spectacles placed in the anterior focal plane of the eye will NOT induce an anisekonia (magnification or minification of the image).
photochromic lenses (transition lenses) work
work by darkening with the absorption of UV light (300-400 nm). Automobiles have a UV absorbing coating that will prevent transition glasses from working properly.
U+P = V
U = object vergence, which is ALWAYS NEGATIVE P = power of lens/mirror V = vergence of light exiting the lens/mirror to form an image
Object and image vergences = reciprocal of their distances from the lens/mirror respectively