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Lens

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The tiny object on this man’s finger is life-changing for him. It lets him see clearly without wearing glasses. You probably recognize the object as a contact lens. You may even wear contact lenses yourself.

What Is a Lens?

A lens is a transparent object with one or two curved surfaces. It is typically made of glass (or clear plastic in the case of a contact lens). A lens refracts, or bends, light and forms an image. An image is a copy of an objected formed by the refraction (or reflection) of visible light. The more curved the surface of a lens is, the more it refracts the light that passes through it. There are two basic types of lenses: concave and convex. The two types of lenses have different shapes, so they bend light and form images in different ways.

Concave Lens

A concave lens is thicker at the edges than it is in the middle. You can see the shape of a concave lens in the Figure below . From the diagram, it’s clear that the lens causes rays of light to diverge, or spread apart, as they pass through it. Note that the image formed by a concave lens is on the same side of the lens as the object. It is also smaller than the object and right-side up. However, it isn’t a real image. It is a virtual image. Your brain “tricks” you into seeing an image there. The light rays actually pass through the glass to the other side and spread out in all directions. You can explore the formation of images by a concave lens at this URL: http://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html

Concave lens

Convex Lens

A convex lens is thicker in the middle than at the edges. You can see the shape of a convex lens in the Figure below . A convex lens causes rays of light to converge, or meet, at a point called the focus (F). A convex lens forms either a real or virtual image. It depends on how close the object is to the lens relative to the focus. You can interact with an animated convex lens at this URL: http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=1395.msg5241#msg5241

Convex lens

Q: An example of a convex lens is a hand lens. Which of the three convex lens diagrams in the Figure above shows how a hand lens makes an image?

A: You’ve probably looked through a hand lens before. If you have, then you know that the image it produces is right-side up. Therefore, the first diagram must show how a hand lens makes an image. It’s the only one that produces a right-side up image.

Summary

  • A lens is a transparent object, typically made of glass, with one or two curved surfaces. A lens refracts light and forms an image.
  • A concave lens is thicker at the edges than it is in the middle. This causes rays of light to diverge. The light forms a virtual image that is right-side up and smaller than the object.
  • A convex lens is thicker in the middle than at the edges. This causes rays of light to converge. The light forms a real or virtual image depending on the distance of the object from the lens.

Vocabulary

  • lens : Transparent object with one or two curved surfaces that forms images by refracting light.

Practice

At the following URL, vary the location of the object in the animation and observe what happens to the image. What can you infer from your observations? http://www.physicsclassroom.com/shwave/lenses.cfm

Review

  1. What is a lens? What does it do?
  2. Describe the image formed by a concave lens.
  3. Explain how a hand lens forms an enlarged image of an object.

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