Some Terms You Should Know: Lenses and Mirrors

These are some terms you will come across in your lab class dealing with lenses and mirrors.
Converging Lens (or Positive Lens)
A lens with a positive focal length which bends parallel rays of light toward a specific point, the focal point.
Diverging Lens (or Negative Lens)
A lens with a negative focal length which bends parallel rays of light away from a specific point, the focal point.
Eyepiece
In a telescope, the last lens before the viewer's eye.
Focal Length
The distance from the center of a lens or mirror to the focal point, the point where parallel rays are focused.
Focus
To take parallel rays of light and bend them so they all meet at one point, the focal point.
Image Distance
The distance between an image and the mirror or lens that produced it.
Inverted Image
An image that is flipped upside down.
Object Distance
The distance between an object and the mirror or lens it is viewed through.
Objective Lens or Objective Mirror
In a telescope, the lens or mirror closest to the object being viewed.
Real Image
An image formed by a lens or mirror that may be projected onto a screen.
Reverted Image
An image that is flipped left-to-right.
Virtual Image
An image that is formed by a lens or mirror that can be seen by looking into the lens or mirror, but not by projection onto a screen. The image in your bathroom mirror (a plane mirror) is a virtual image.


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This page was last updated on 18 January 1998
and is © 1998 by Jim McDonald.