Concave and convex lenses are used to bend the light rays which will change the focal point - point of focus. They are used in eyeglasses, contacts, microscopes, telescopes and cameras etc.
How does a mirror reflect light? The mirrors that you use everyday, the ones you have in your homes, have flat surfaces. Because of this, they reflect almost all of the light. The image you see, the virtual image, the one in the mirror is the same size as the real object. The virtual image appears to be behind the mirror. The distance between the mirror and the object is the same as the distance between the virtual image and the mirror. The real object and the virtual image are basically identical except for one thing - the virtual image is reversed.
If you have ever noticed the mirrors that hang in stores, you would have realized that not all mirrors have flat surfaces. There are curved mirrors. These mirrors also refect light, but because they are curved the virtual image will look different. Curved mirrors produce the effect of enlarging or reducing. There are two types of curved mirrors: concave and convex. A spoon is a good example of a refecting surface with one side being convex the other concave. Concave increases the size of the object while convex decreases the size.
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convex mirror
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concave mirror
light
light
What exactly are lenses? A lens is a transparent material, such a glass, that has either one curved surface and one flat surface or two curved surfaces. As with mirrors, these two lenses are either convex or concave. Convex lenses are thicker in the middle then the edges and concave are thicker at the edges then the middle. When light travels through lenses, refraction occurs. The light bends either outward or inward, it depends on the lens.
The lens of your eye is a double convex lens. Its job is to focus the image on the retina of the eye. If one is farsighted, the lens in the eye causes the focus to be behind the retina. These people see far but have some difficultly seeing close-up. The lens focuses behind the retina because the actual eyeball is too short from front to back. To correct this farsightedness, the person would wear glasses or contacts with convex lenses. What would be the result and treatment if a person's eyeball was too long from front to back? (answer is at the bottom)
NEAR SIGHTED Corrective
Lense
Your eye and a camera are quite similar. Both use convex lenses to focus the image upside-down. Your eye uses its retina to detect the image whereas the camera uses film to detect the image.
If one is nearsighted, the lens in the eye causes the focus to be behind the retina. These people see close-up but have some difficultly seeing far away. The lens focuses infront of the retina because the actual eyeball is too long from front to back. To correct this nearsightedness, the person would wear glasses or contacts with concave lenses. Take a look at the two diagrams.
