Optics and Refractive states of the eye

refractive state

the ability of the eye (lens, cornea etc.) to bring objects into focus.

converge

come together. Light rays passing through glass or plastic will converge at some point on the other side.

When a bundle of rays are brought together, they are said to

converge

The power of a lens is measured by it's ability to....

converge or diverge light rays. This ability depends on the refractive index of the lens material and the shape of the lens.

presbyopia

diminished accommodation due to aging that primarily affects near vision.

Lens power is considered in terms of focal length (the distance between the focal point and the lens) as follows: the power of a lens is equal to the reciprocal of the focal length measured in meters and expressed in units called _____.

diopters

Rays of light from a luminous point of light will spread out. This is

divergence

refractive state in which parallel light rays are focused perfectly on the retina is called ______.

emmetropia. In the normal eye light rays from a distant object are focused sharply on the retina with a relaxed lens. For near objects light rays are divergent and the eye has to accommodate to see those up close objects clearly (because light rays aren't parallel anymore).

concave lens

lens that is curved inward. (will always curve in direction of the base). These lenses are known as negative or minus lenses. Light rays diverge after refraction and therefore don't focus behind lens. There is an imaginary focal point (virtual) that is in front of the lens.

convex lens

lens that is curved outward. Light rays converge on the other side of the lens along the principal axis. These lenses are known as plus or positive lenses. A more curved lens will bend rays of light to a shorter focus. Focal point will be behind lens.

Incident ray

light ray entering a medium

fusion

merging images seen from two eyes into one image

plano

no sphere

light rays are assumed ______ if they emanate from a distant light source.

parallel

parallel light rays that enter the lens at any point other than the center are known as ________ rays.

paraxial.

emergent ray

ray emerging from a medium

Prism

used to correct diplopia (double vision.)

refraction

Bending of light rays. Light travels at different speeds depending on the various substances it travels through.

hyperopia

farsighted. Cornea and lens have too little plus power . Light rays are focused behind the retina.

Light intensity is measured in terms of

footcandles

Cylinder in lens focuses light rays to a focal ___ rather than a point.

line. The orientation of the cylinder is indicated by the axis which ranges from 0 (horizontal) through 90 (vertical) to 180 (same as 0). Generally spherocylinders are used in lenses.

Spherical convex

+ lens. Used to provide clear near and distance vision for hyperopic individuals. Also used to provide near vision for emmetropic individuals w/presbyopia (reading glasses).

spherical concave

-lens. Used to provide clear distance vision for those with myopia. When myopes develop presbyopia they may only need to remove their glasses to read as their near vision is good.

myopia

Also known as nearsighted. The cornea and lens of the non-accommodating eye have too much plus power. The parallel light rays fall in front of the retina and distant images can appear blurred.

spherocylinders

Also sometimes called a toric lens. Can be thought of as an object cut from a football. Used in astigmatism.

Sphere lens has _____ over it's entire surface.

Curvature. Has same refractive in all directions. Focuses light rays to a single point. May be concave or convex.

focal length

Distance from focal point to center of lens. A lens is considered in terms of it's focal length and expressed in terms of diopters.

With concave lenses the paraxial rays diverge behind the lens and therefore cannot be focused behind the lens. Where is the focal point and what is it called?

It's in front of the lens. It's "imaginary" and called the virtual focal point.

Optics

Physical science that deals with the properties of light and vision.

focal point

With convex lenses, paraxial rays from a distant source are refracted by the lens material and converge at a point somewhere along the principal axis behind the lens. This point of converging is known as the focal point.

what causes myopia?

a "longer" eye shape than the average eye or the cornea has too much curvature. Distant light rays are not focused on the retina as a result.

what causes hyperopia?

a "shorter" eye shape than average and too little power of the lens and cornea (not enough curvature.) Hyperopic eyes can see better at a distance than near but distance is also a little blurred as parallel light rays don't focus properly on the retina.

refractive state in which parallel light rays are not focused perfectly on the retina due to refractive errors is called ____

ammetropia.When a relaxed eye cannot focus distant light rays on retina the eye is ammetropic. Three conditions produce this: myopia, hyperopia, and astigmatism.

If a light ray strikes the center of a lens and is perpendicular to the lens it is known as the _______ axis.

principle axis. The ray is called the principal ray.

Transposition

process of changing prescription from plus cylinder to minus or vice versa. Equation is as follows: 1. add algebraically the cylindrical power to the spherical power. 2. reverse the sign of the cylinder from plus to minus or vice versa. 3. add or subtract 90' to make the new axis 180' or less.

diverge

spread apart. If a prism is placed together apex to apex (top to top) the light rays will diverge on the other side.

accommodation

the lens of the eye can change shape to change is "power" of refraction. This process is called accommodation.