4.5.2.3b - Vision Defects And Their Correction

Focus And Refraction

The retina is the light-sensitive layer at the back of the eye. For clear vision, light rays from an object must be refracted so that they focus on the retina.

The eye refracts light mainly at the cornea and the lens. For this lesson, the important target is the retina: if rays focus in front of the retina or behind it, the person does not see the object clearly.

You may already have met accommodation: the way the eye normally changes lens shape for near and distant objects. Here, the question is different: what happens when rays still do not focus on the retina, and how correction lenses fix that?

Myopia: Short Sightedness

Myopia is short sightedness. A person with myopia can usually see close objects more clearly than distant objects. Distant objects are blurred because light rays focus in front of the retina instead of on it.

This can happen if the eyeball is too long from front to back, or if the eye lens is too thick or refracts light too strongly. In either case, the rays meet too soon.

Myopia is usually corrected with a concave spectacle lens. A concave lens is thinner in the middle than at the edges. It refracts the incoming rays so they are spread out slightly before they enter the eye. The eye then brings the rays to a focus further back, on the retina.

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Hyperopia: Long Sightedness

Hyperopia is long sightedness. A person with hyperopia usually has more difficulty seeing nearby objects clearly than distant objects. Nearby objects are blurred because light rays would focus behind the retina instead of on it.

This can happen if the eyeball is too short from front to back, or if the eye does not refract the light strongly enough. In either case, the rays have not met by the time they reach the retina.

Hyperopia is usually corrected with a convex spectacle lens. A convex lens is thicker in the middle than at the edges. It refracts the incoming rays so they start converging before they enter the eye. The eye then brings the rays to a focus sooner, on the retina.