First of all we have to know what is the ideal refractive condition of the eye. The ideal refractive condition of the eye is known as emmetropia. So emmetropia may be defined as the refractive condition of the eye where parallel rays of light from infinity come to a focus on the retina, better to say on the fovea when accommodation is at rest. Any variation from this standard constitutes what is known as ametropia. So ametropia may be defined as the refractive condition of the eye where due to some refractive error, parallel rays of light from infinity do not come to a focus on the retina, better to say on the fovea when accommodation is at rest.
Types of refractive errors of the eye -
Hypermetropia-It is a type of refractive error of the eye where parallel rays of light from infinity come to a focus behind the retina when accommodation is completely relaxed. It is also called long-sightedness. Divergent rays from a near object are focused still further back.
Treatment- Treatment is by spherical convex lens. Ordinary spectacles may be used or contact lenses may also be used.
Myopia-It is a type of refractive error of the eye where parallel rays of light from infinity come to a focus in front of the retina when accommodation is at rest. It is also called short-sightedness.
Treatment- Treatment is by spherical concave lens. Ordinary spectacles may be used or contact lenses may also be used.
Astigmatism- It is a type of refractive error of the eye where parallel rays of light from infinity cannot converge to a point focus due to unequal refractions in different meridians of the optical system of the eye but form focal lines. Astigmatism may be regular where the refractive power changes uniformly from one meridian to the other. Astigmatism may also be irregular where the refractive power changes irregularly in different meridians of the optical system of the eye.
Treatment- Regular astigmatism may be treated both by ordinary cylindrical lenses or by contact lenses. But irregular astigmatism requires the use of contact lenses only.
Types of refractive errors of the eye -
Hypermetropia-It is a type of refractive error of the eye where parallel rays of light from infinity come to a focus behind the retina when accommodation is completely relaxed. It is also called long-sightedness. Divergent rays from a near object are focused still further back.
Treatment- Treatment is by spherical convex lens. Ordinary spectacles may be used or contact lenses may also be used.
Myopia-It is a type of refractive error of the eye where parallel rays of light from infinity come to a focus in front of the retina when accommodation is at rest. It is also called short-sightedness.
Treatment- Treatment is by spherical concave lens. Ordinary spectacles may be used or contact lenses may also be used.
Astigmatism- It is a type of refractive error of the eye where parallel rays of light from infinity cannot converge to a point focus due to unequal refractions in different meridians of the optical system of the eye but form focal lines. Astigmatism may be regular where the refractive power changes uniformly from one meridian to the other. Astigmatism may also be irregular where the refractive power changes irregularly in different meridians of the optical system of the eye.
Treatment- Regular astigmatism may be treated both by ordinary cylindrical lenses or by contact lenses. But irregular astigmatism requires the use of contact lenses only.
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