Physics for Kids
Lenses and Light
A lens is a curved piece of glass or plastic designed to refract light in a specific way. Lenses are used in glasses and contacts to help correct vision. They are used in telescopes to help view items that are far away and are used in microscopes to help view very small items.
When a light wave moves from one medium (like air) to another medium (like glass) the light rays are bent. This is called refraction. By using refraction, lenses can bend multiple light rays. Most of the lenses we use in everyday life are designed to bend light rays to a specific focal point where items will be in focus (clear).
You can go here to learn more about the refraction of light.
Types of Lenses
There are different ways to classify lenses. One way to classify lenses is by how they bend light.
A converging lens will cause the light rays to bend to a specific focal point. Another name for this type of lens is a positive lens.
A diverging lens will cause light rays from a specific focal point to be spread out. Another name for this type of lens is a negative lens.
Other Types of Lenses
Another way to classify lenses is by the curve of the glass on each side of the lens. There are terms used to describe each side. Then the two sides are combined to come up with the name of the lens.
- Convex - A convex lens is one where the center of the lens is thicker than the edges.
- Concave - A concave lens is one where the center of the lens is thinner than the edges. One way to remember the difference between the two lenses is to think of "caving in" with the concave lens.
- Plano - A plano lens is a flat lens. This is used when one side is flat and the other side is concave or convex. You can think of flat as a "plain."
- Meniscus - A meniscus lens is one where one side is concave and one side is convex.
- Biconvex - A lens in which both sides are convex is biconvex. Biconvex lenses are converging lenses.
- Plano-convex - A lens in which one side is convex and the other is plano. Plano-convex lenses are converging lenses.
- Biconcave - A lens in which both sides are concave is biconcave. Biconcave lenses are diverging lenses.
- Plano-concave - A lens in which one side is concave and the other is plano. Plano-concave lenses are diverging lenses.
- Positive meniscus - A converging lens where one side is concave and the other convex.
- Negative meniscus - A diverging lens where one side is concave and the other convex.
The focal point of a lens is generally noted by the capital letter "F." This is the point in space where the light rays will converge to after passing through a converging lens. A diverging lens will have a negative focal point where the rays originate from before diverging through the lens.
The focal length is the distance from the center of the lens to the focal point.
The principal axis is a horizontal imaginary line drawn through the center of the lens. In a perfect lens the focal point will reside on the principal axis at a distance of the focal length from the center of the lens.
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