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| A piece of glass (or other transparent material) that can bend parallel rays of light so that they cross, or appear to cross, at a single point. |
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| A lens that is thickest in the middle, causing parallel rays of light to converge to a focus. Also known as a convex lens. |
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| A lens that is thickest in the middle, causing parallel rays of light to converge or focus. Also known as a converging lens. |
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| A lens that is thinnest in the middle and that causes parallel rays of light to diverge. Also known as a concave lens. |
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| A lens that is thinnest in the middle and that causes parallel rays of light to diverge. Also known as a diverging lens. |
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| The line joining the centers of curvature of the surfaces of a lens. |
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| For a converging lens, the point at which a beam of light parallel to the principal axis converges. For a diverging lens, the point from which such a beam appears to come. |
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| A plan passing through either focal point of a lens that is perpendicular to the principal axis. For a converging lens, any incident parallel beam of light converges to a point somewhere on a focal plane. For a diverging lens, such a beam appears to come from a point on a focal plane. |
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| The distance between the center of a lens and either focal point. |
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| An image that is formed by converging light rays and that can be displayed on a screen. |
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| A diagram showing rays that can be drawn to determine the size and location of an image formed by a mirror or lens. |
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| Lens of a telescope closest to the eye; enlarges the real image formed by the first lens. |
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| In an optical device using compound lenses, the lens closest to the object observed. |
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| The transparent covering over the eyeball. |
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| The colored part of the eye that surrounds the black opening through which light passes. The iris regulates the amount of light entering the eye. |
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| The opening in the eyeball through which light passes. |
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| The layer of light-sensitive tissue at the back of the eye. |
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| Term applied to a person who has trouble focusing on nearby objects because the eyeball is so short that images form behind the retina. |
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| Term applied to a person who can clearly see nearby objects but not clearly see distant objects. The eyeball is elongated so that images focus in front of rather than on the retina. |
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| A defect of the eye caused when the cornea is curved more in one direction than in another. |
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| Distortion in an image produced by a lens. |
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