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Physics of the Eye and Intralase Surgery. Sight and lenses. The eye is a series of lenses that refract and focus light. These lenses follow the same laws of physics that artificial lenses would. Lenses and focal points.

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sight and lenses
Sight and lenses
  • The eye is a series of lenses that refract and focus light.
  • These lenses follow the same laws of physics that artificial lenses would.
lenses and focal points
Lenses and focal points
  • The cornea and lens of the eye act as two convex lenses. In order to understand illness and disorders of the eye, we must understand how lenses and focal points work.
lenses and focal points1
Lenses and focal points

To find the focal point, distance of an image, or distance to the object you can use

the thin lens equation.

Light bounces off an object

from a distance s1. This light

passes through the lens to be

focused on the other side. This

image is real, inverted, and

smaller, and is located at a

distance of s2 from the lens.

The thin lens equation

lenses and focal points2
Lenses and focal points

Focal Point

A convex lens

lenses and focal points3
Lenses and focal points
  • Changing the distance to an object, or changing the focal length of a lens, will result in a different focal point.
  • Changes of these kind in the eye are the causes of visual acuity loss.
sight it s a light thing
Sight, it’s a light thing.
  • Light is reflected off of objects and enters into the eye.
  • Colors are the wavelengths that are not adsorbed by the substance.
anatomy and function of the eye
Anatomy and Function of the Eye
  • As light enters the eye, it first passes through the cornea. Most of the refraction of light occurs here. The refractive index of the cornea is 1.38.

Refractive index=

Speed of light in vacuum

Speed of light in substance

anatomy and function of the eye1
Anatomy and Function of the Eye
  • The eye can control the amount of light that enters by the opening and closing pupil aperture using the iris.

<- Dilated

Constricted ->

anatomy and function of the eye2
Anatomy and Function of the Eye
  • After passing through the cornea, light enters and refracts again through the lens of the eye. The lens has a refractive index of 1.44.
anatomy and function of the eye3
Anatomy and function of the eye

This also changes the bulge of the

cornea somewhat. This also aids to

adapt vision into different situations.

The eye can change the shape of the lens

using muscles called ciliary muscles that

stretch and flatten the lens, or relax and

allow it to return to normal shape. This is

called accommodation.

Sometimes this bulging of the cornea is an appreciable change that can readily be seen

by the naked eye.

anatomy and function of the eye4
Anatomy and Function of the Eye
  • The light then is focused onto the fovea, a cone and rod dense area on the retina, that produces the sharpest visual image.
vision disorders
Vision Disorders

Normal eye. Focus hits directly

on fovea to produce sharp image.

vision disorders1
Vision Disorders

This is known as nearsightedness, or myopia. The image comes into focus before

the retina. Things that are near to the eye can more easily be seen. This

condition can result from the eye being too long, the cornea being too round,

or the lens having too much focus power.

vision disorders2
Vision Disorders

This condition is known as farsightedness, or hyperopia. The image produced by the

lens and cornea comes into a focal point behind where the retina is located. Things

that are farther away can be more easily seen. This results from the eye being too

short, the cornea being too flat, or the focus power of the lens being too weak.

vision disorders3
Vision Disorders

This is a condition known as astigmatism. The shape of the lens and/or cornea is

irregular leading to a loss of focal points or, more commonly, two focal points on

the retina.

vision disorders4
Vision Disorders

This condition is known as presbyopia. The lens becomes rigid and fails to bend

or accommodate to changing conditions.

  • For centuries people have been using lenses to correct or compensate for vision loss. Traditional remedies are as follows:

A diverging lens will correct

for nearsightedness.

A converging lens corrects for


Bifocal lenses will compensate for


A custom ground lens made specifically

for each eye will correct astigmatism.

  • More recently, doctors have been using surgery to correct vision problems. These surgeries employ the same physics as corrective lenses would outside the eye, only the changes are done to the cornea and lens of the eye itself.
  • Radial Keratotomy was one of the original vision correction surgeries.
  • Small cuts were made in the cornea radiating from the center. This was
  • used to treat mild cases of myopia.
  • Automated lamellar keratoplasty used a
  • microkeratome to make a small flap in the cornea. This was then folded out of the way, and the microkeratome was then used to sculpt the cornea. ALK can only correct myopia.

A microkeratome


Photorefractive keratotomy is the precursor to LASIK surgery.

This type of surgery was introduced in 1987. It uses a Eximer laser

to reshape the outside of the cornea and thus correct vision. PRK can correct

myopia, astigmatism, and some hyperopia.

  • In 1989 the FDA approved LASIK (Laser-Assisted in Situ Keratomileusis)
  • for treatment of:
  • Nearsightedness
  • Farsightedness
  • Astigmatism

In LASIK surgery a mechanical

cut is made across the cornea to

create a flap of tissue and exposing

the stroma of the cornea.

This flap is moved aside and a “cool ultraviolet” laser

sculpts and re-shapes the cornea, correcting acuity

problems. This laser allowed for greater precision and

thus greater flexibility for treatment of a broader

range of symptoms.

intralase surgery
Intralase Surgery

In 2001, the FDA approved a laser to be used in a surgery called Intralase.

Before surgery, exhaustive measurements are taken of each eye that will be operated on. Measurements of refractivity of the eye tell the prescription that each cornea and lens is contributing.

The topography of the cornea and the measurements of the pupil are also taken.

intralase surgery1
Intralase Surgery

The eye is anesthetized with drops and a suction ring is placed on the cornea to immobilize the eye. A guide cone is then placed on the ring to direct the eye appropriately. The laser will track the eye in case of accidental eye movement during the operation.

intralase surgery2
Intralase Surgery

A femosecond laser then makes a very precise

(+/- 10 microns) flap across the surface of the cornea,

instead of a microkeratome. (A human hair is 50

microns in diameter). This reduces rare complications

that LASIK presented, and also opened corrective

eye surgery to previously ineligible patients.

This laser produces carbon dioxide and water bubbles in the cornea. These bubbles

are what actually produce the “incision” to create the flap. This can also be done vertically to produce a deeper incision.

intralase surgery3
Intralase Surgery
  • After waiting for the cavitation bubbles to disappear, the flap is pulled aside and the cornea is then sculpted to the needed measurements.
intralase surgery4
Intralase Surgery
  • The flap is then folded back, and the patient is ready to return home.
  • A few follow up appointments are done to ensure the eyes are moist and that the flaps are closed properly.
intralase surgery5
Intralase Surgery
  • Intralase and LASIK surgeries can repair hyperopia, myopia, and astigmatism.
  • By sculpting one eye to be “farsighted” and one to be “nearsighted”, you can eliminate the need for bifocals as well.
intralase surgery6
Intralase Surgery
  • Intralase surgery has the same benefits of LASIK, but with fewer risks by using a laser to make the flap instead of a mechanical cut.
  • Greater precision also allows patients that were at too high of a risk for complications before to now be eligible for surgery.