THE EYE AS AN OPTICAL SYSTEM Flashcards
****The Visual system is the part of the?
Central Nervous System
Visible Light
It builds a representation of the world around us
First, white light is reflected off an object (the images we see are made up of light reflected from the objects we are looking at).
just read
****First light passes through the
Tear Film on the surface of the eye
****Second then through the
Cornea
****Third to the
Anterior chamber
****fourth then the light passes through the
Iris
The anterior chamber is filled with a substance called
aqueous
Whats the iris?
The iris is the colored part of the eye that controls the amount of light entering the eye.
what changes the size of the iris
Pupil
what happens when theres a lot of light incoming
The pupil will be small
Low light situations
Pupil will be larger Dialated
****The fifth is the
pupil
*****The sixth Step
INVERTED
After going through the iris, the light passes through the lens. The light (image) becomes inverted.
****The light proceeds down the _______ through the_______ to the ________.
- Visual Axis
- Vitreous
- Retina
****The Visual Axis
Is the path of light that starts the object being viewed and ends at the FOVEA
Where is the Fovea
Center of the macula
Where is the Retina and what does it do and contains
The retina lines the back of the eye and contains photoreceptor cells which convert this light into electrical impulses.
Where are electrical impulses transmitted from? Where does it start and finish
optic nerve which emerges from the back of the eye and extends into the brain.
As the retinal fibers leave the optic nerves, half of them (from each eye) cross over to the opposite side at the
Optic Chiasm
The fibers fan out into the posterior portion of the brain called the______. It is in this area of the brain that conscious recognition of visual impulses takes place and an image is “seen”.
Visual Cortex
******The human eye has 5 optical media?
What are the 5 optical media
Tears, cornea, aqueous, lens, vitreous
The light is
refracted at the interfaces between each of these media. The goal is to have a focused image before it lands on the retina.
Fibers then form the ______ Which continue towards a cell station in the brain called the ______
Optic tracts Lateral geniculate body
The tear is composed of 3 layers
- Outermost lipid or fatty layer
- Central layer is most aqueous
- Mucous layer
The tear film serves several functions
***** Its forms a smooth refractive surface on the epithelium by abolishing minute surface epithelial irregularities
* It maintains a moist environment for the epithelium (cornea) and protects it’s delicate surface
* It provides the cornea with nutrients
*it inhibits growth of microorganisms by mechanical flushing and antimicrobial action
Cornea
The cornea is a clear, transparent, dome shaped structure with a shiny surface. It has a convex shape that acts as a powerful lens. Most of the refraction of they eye takes place through the tears and cornea, about 75%.
The cornea has 5 distinct layers
- The epithelium
- Bowman’s layer
- The stroma
- Descemet’s membrane
- The endothelium
The epithelium functions as a barrier and as an important refractive optical surface. It regenerates rapidly and heals without leaving a scar. Why is this so important to humans??
read
Aqueous
The aqueous is a clear liquid that fills the anterior and posterior chambers of the eye. It is similar in composition to plasma. It is produced by the ciliary body. It enters the posterior chamber, passes through the pupil into the anterior chamber and leaves through the trabecular meshwork. It helps maintain the shape and pressure of the eye.
Crystalline Lens
The lens of the eye is a transparent, biconvex structure situated between the iris and the vitreous. It is suspended behind the iris by zonules. The lens is surrounded by a capsule, which is a transparent, highly elastic envelope. It consists of about 65% water, about 35% protein, and a trace amount of minerals. It refracts light by changing it’s shape.
Refraction
of light occurs at each of these interfaces, and the total of these refractions helps to establish the refractive status of the eye (how well the eye can focus images onto the retina).
Refraction occurs
because the optical structures of the eye each have different IRs as well as different curvatures.
which one goes up or down
air to tears????
Light going from air to tears lower n –> higher n Light going from tears to cornea lower n –> higher n Light going from cornea to aqueous higher n –> lower n Light going from aqueous to lens lower n –> higher n Light going from lens to vitreous higher n –> lower n
The refracting power of a curved surface can be calculated by the following formula
P = (n2-n1) r(m)
Refractive surgeries (PRK, LASIK, LASEK, SMILE) aim to change the refractive state of
the eye by increasing or decreasing the radius of curvature of the anterior cornea.
Increase the r = decrease the refractive power of the eye
Reduce the r = increase the refractive power of the eye
The 2 main factors that determine the power of a lens and thus its ability to refract light:
The index of refraction (density of the medium)
The curvature (radius of curvature)
The distance from the center of a lens to the focal point is the
focal length
The power of a lens is measured in
Diopters (D)
The power is related to the focal length.
A 1 D lens will bend parallel light rays to a focal point that is 1 meter from the lens.
D means F means
D = the dioptric power of the lens F = focal length of the lens in meters
whats the formula
D = 1/F or F = 1/D
D = 1/F So if the focal length is 2 m ½ = 0.50 D
F = 1/D So if the power is 4.00 D ¼ = 0.25 m
If other units of measurement are used…
D = 1/F meters D = 100/F centimeters D = 1000/F millimeters D = 40/F inches
A lens has a focal length of 4 inches. What is the power of this lens?
D = 40/F D = 40/4 D = 10 Diopters
A lens has a focal length of 12 mm. What is the power of this lens?
D = 1000/F D = 1000/12 D = 83.33 Diopters
A lens has a power of 15 D. What is the focal length in cm of this lens?
15 = 100/F 15(F) = 100 F = 100/15 F = 6.67 cm
light rays are moving away from each other. Light rays emanating from a source of light are divergent.
Divergent
Light rays moving toward each other are convergent. Convergent light rays usually do not occur in nature but are the result of the action of an optical system (eg, a lens).
Convergence is expressed in plus power. Therefore, a lens that causes convergence is expressed in (plus) red.. power (+).
The power of a lens is typically rounded to 2 decimal places. Usually in 0.25 increments but sometimes you will see them in 0.12 D increments. 1.25
The sign (whether plus or minus) must always be noted if there is a power. +/- +1.25 -1.25
Do we need plus or minus power to converge light rays?
Therefore, the overall power of the eye must be plus.
Here, the vergence and power changes provided by the 5 curved surfaces of the eye are summarized
picture slide 59
Why do patient’s with dry eyes complain of blurry vision?
The greatest plus power in the eye is provided by the tear-cornea curved surface. When the surface is disrupted, a substantial amount of plus power is lost, resulting in a blur.
****How does the process of vision occur?
- WHITE LIGHT reflected off an object
- Light Passes through TEAR FILM
- Then CORNEA
- ANTERIOR CHAMBER
- Anterior chamber has AQUEOUS
- IRIS
- PUPIL
- LENS
9.
*****Which part of the eye has the greatest refracting power?
Most of the refraction of they eye takes place through the tears and cornea, about 75%.
****Radius Curvature?
The other variable we need to look at is radius of curvature (r) in meters (though commonly written in mm).
Spheres with a shorter radius (a smaller sphere) have base curves that are higher (or steeper) than spheres with a longer radius (a larger sphere) which are flatter.
****Factors that determined the power of a lens
The refracting power of a curved surface can be calculated by the following formula…
P = (n2-n1)
r(m)
The 2 main factors that determine the power of a lens and thus its ability to refract light:
- The index of refraction (density of the medium)
- The curvature (radius of curvature)
****Light going through center of lens is not refracted?
In any lens, the ray of light going through the center of the lens is not refracted. But all rays on either side will be refracted. They will either converge (come together) to form a point or diverge from a point (more on this later). This is known as the focal point.
****Focal point
.
They will either converge (come together) to form a point or diverge from a point (more on this later). This is known as the focal point.
****Focal point
.
They will either converge (come together) to form a point or diverge from a point (more on this later). This is known as the focal point.
*****Focal length?
The distance from the center of a lens to the focal point is the focal length
****Calculating the focal length or dioptric power of a lens?
The power of a lens is measured in diopters (D).
The power is related to the focal length.
A 1 D lens will bend parallel light rays to a focal point that is 1 meter from the lens.
D = 1/F or F = 1/D
D = the dioptric power of the lens F = focal length of the lens in meters
****Focal length in relation to lens power?
Lenses with shorter focal lengths have higher powers
longer focal lengths have lower powers
***Convergence
Light rays moving toward each other are convergent. Convergent light rays usually do not occur in nature but are the result of the action of an optical system (eg, a lens).
Convergence is expressed in plus power. Therefore, a lens that causes convergence is expressed in plus power (+).
The power (or vergence power) of a lens describes its ability to converge or diverge light rays.
****Divergence
Divergent light rays are moving away from each other. Light rays emanating from a source of light are divergent.
Divergence is expressed in minus power. Therefore, a lens that causes divergence is expressed in minus power (-).
****The power of the eye?
How much power does the eye have?
, the overall power of the eye must be plus.
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