Q:

# How do optical lenses work?

A:

Optical lenses use the principle of refraction to focus light rays or disperse them. Refraction is the bending of a ray of light when it moves between media of different densities at an angle.

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Optical lenses have a higher density than the air that surrounds them. When light travels from air to the denser medium of the lens, it bends and either focuses on the other side of the lens or disperses, depending on the shape of the lens. Lenses which cause light to focus on one point are called converging lenses. The focal length of the lens is the distance between the center of the lens and the point where the light focuses. Lenses which cause parallel light rays to diverge are called diverging lenses. The focal length of any lens is directly proportional to the radius of curvature of the lens. A flatter lens has less of a curve and therefore a higher radius and higher focal length and can focus light at a greater distance.

The lens of the human eye is designed to converge light onto the retina. The focal length of the eye lens can be changed by the muscles that control it. The focal length is increased when the muscles flatten the lens. This allows for visualization of distant objects. The focal length is decreased when the lens is more curved and this allows humans to see objects that are near. Contact lenses or eye glasses are typically worn when the lens of the eye is not able to focus the images on the retina correctly, because they are focused either behind or in front of the retina. This requires an additional lens to refract the light correctly.

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