Objective: To describe principles of corrective lenses

• Correction is the process of using optical devices (spectacles or contact lenses) to bend rays of light entering the eye to obtain the best visual acuity.
• Correction is generally performed by optometrists or ophthalmologists. A teacher cannot prescribe either spectacles or contact lenses, because he has neither the training nor the equipment. However, he can recognize the need for correction based on simple vision tests or an FVA (see Module 4).
• Changing the direction of a light ray is called refraction. A convex or plus lens causes the rays to move together. A concave or negative lens causes the rays to move apart.
• Magnifiers are convex lenses that make the image larger. The person using the magnifier must move closer to it to focus the image. If an image appears four times larger than the actual image, the magnification is 4x. The power of lenses is measured in diopters; the stronger the lens, the greater the number of diopters.
• A spectacles prescription usually consists of a separate measurement for each eye. Some prescriptions may include both distance and near point values. If ADD with a number is included on the prescription, it indicates that additional power is added to the lower half of the lens (a bifocal) to assist with near vision tasks such as reading.