THE HUMAN EYE

SUAAD MOHAMMADERIN OLSON

Refraction and Converging Lenses

Refraction

Refraction is the way that light bends when going from one medium to another. Converging lenses are types of lenses where the light enters the lens, and refracts towards the normal. An example of a thin converging lens is the human eye. In the human eye, the light comes in, and bends towards the retina, where the picture is formed. The picture that is formed is an inverted picture. The human brain must then flip the image formed in order for us to see the world right side up.

The Normal Human Eye

The human eye is a convex lens in which rays of light converge. Light is refracted on the cornea and lens, and meets on the back of the retina, creating a clear picture.

Nearsighted

Nearsighted vision is when the light focuses in front of the retina. There can be different causes for this problem.

1.1. The eye is too long and the light focuses before hitting the retina.

2.2. Astigmatism – The cornea is not perfectly spherical causing light to refract at a different angle.

3.3. Curvature – If the lens is steeply curved it causes nearsightedness.

Farsighted

Farsightedness takes place when the light is not completely focused before hitting the retina. The reasons for this condition are opposite for those of nearsightedness.

1.1. The eye is too short causing the light to focus beyond the retina.

2.2. Astigmatism – The cornea is not spherical causing the light refraction to be off.

3.3. Curvature – If the lens is too flat, it causes farsightedness.

Refraction Test

If you are having trouble with your vision, an eye doctor will perform what is called a refraction test to determine if there is a need for correction. During this test, an eye doctor will use different lenses to refract the light differently to determine if one of the lenses helps you to see more clearly. If correction is needed, they may write you a prescription for glasses or contacts.

Corrective Lenses

To correct a vision problem, eye glasses can be prescribed. The lens works to refract the light in such a way that focuses the light correctly on the back of your retina to create a sharper image. Contact lenses are used in the same fashion.

Lasik Eye Surgery

Another way to correct vision in a more permanent way than with contacts or glasses is LASIK Eye surgery. During the LASIK procedure, the doctor uses a laser to reshape the cornea. To do this, the doctor must first cut the flap on top of the cornea to reshape it. An alternative to the LASIK is the photorefractive keratectomy or PRK. This procedure also uses a laser to reshape the cornea, but does not peel back the flap, but scratches the surface. Both procedures work for patients who are nearsighted, farsighted or have astigmatism.

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Sources Used