Human Eye Anatomy, Optical Instruments, and Vision Defects

Anatomy of the Human Eye

The human eye is shaped like a sphere with a diameter of about 25 mm. It consists of the following components:

• Sclera: The outer membrane of the eye.
• Cornea: The front part of the sclera. It is shaped like a meniscus or a plano-convex converging lens.
• Aqueous Humor: Separates the cornea from the crystalline lens.
• Choroid: An inner membrane surrounding the interior of the eye.
• Iris: The anterior part of the choroid.
• Pupil: A small hole located at the center of the iris through which light rays enter. It regulates the entry of light according to its intensity.
• Ciliary Muscle: Modifies the geometry of the lens, changing its optical power.
• Retina: Composed of many photoreceptor cells (rods and cones), it is where light rays are projected to form an image.
• Fovea: The area with the highest concentration of rods and cones, where the eye naturally points so that a sharp image is formed.
• Vitreous Humor: Its function is to maintain the elastic properties of the eye; it has an index of refraction similar to water.

Accommodation of the Lens

This is an involuntary process in which the ciliary muscles vary the focal length of the crystalline lens according to the distance of the object being viewed.

Optical Instruments

Magnifying Glass (Lupa)

A converging lens of short focal length (and high power) placed between the eye and the object to increase the size of the image formed on the retina.

Compound Microscope

This consists of two converging lenses: the objective lens (closest to the object, with a smaller focal length) and the eyepiece (ocular) lens (closest to the eye, with a slightly greater focal length).

Telescopes

These provide inverted images. A terrestrial or astronomical telescope consists of a converging lens (objective) with a long focal length and a diverging lens (eyepiece) with a smaller focal length, separated so that the optical interval is null.

Common Vision Defects and Corrections

Myopia (Nearsightedness)

In a myopic eye, the image of a distant object is formed in front of the retina, making it blurry. Nearby objects can be seen clearly because their images project correctly onto the retina. Thus, a myopic person sees well up close but poorly at a distance. To correct this, a diverging lens is used to reduce the excess refractive power of the eye's lens.

Hyperopia (Farsightedness)

A hyperopic eye is insufficiently convergent. Through accommodation, the eye can focus the image of distant objects onto the retina, but it lacks the power to do so for nearby objects. Thus, farsighted people see well at a distance but blurry up close. Their near point is further away than normal. To correct this, a converging lens is used to increase the overall refractive power.

Presbyopia

This is the reduction of the eye's accommodation capacity due to ciliary muscle fatigue or loss of flexibility in the crystalline lens. For a presbyopic eye, the far point does not change, but the near point recedes. It is corrected using bifocal or progressive lenses.

Astigmatism

A visual defect that occurs because the cornea is not perfectly spherical, causing the image of a point to appear as a line or stroke. It hinders the clear, simultaneous visibility of perpendicular lines (such as the spokes of a bicycle wheel). It is corrected using cylindrical lenses of variable power.