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Concave Mirrors

Concave mirrors focus light and create reflected images are of different sizes than the original object.

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Distorted Images

Distorted Images

Credit: Flickr: Eden, Janine and Jim
Source: http://www.flickr.com/photos/edenpictures/2209366634/
License: CC BY-NC 3.0

Fun house mirrors distort images by either stretching or shrinking the image of the object placed in front of them. As the picture shows, the child's reflection is elongated. Knowing this, can you guess whether it is a concave or a convex mirror?

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Credit: Flickr: SBA73
Source: http://www.flickr.com/photos/7455207@N05/4463257694
License: CC BY-NC 3.0

Convex mirrors create a wide effect [Figure2]

  • The distortion that is seen when standing in front of fun house mirrors is determined by the angle of the mirror. If the mirror is bulged outward, it is known as a convex mirror. Convex mirrors make the object look shorter and wider than it really is. If the mirror is bent inward, it is a concave mirror. This type of mirror makes the object look taller and wider than it really is.
  • Spherical mirrors, such as concave and convex mirrors, form the image at a spot called the focal point. The distance from the focal point is called the focal length and is defined by


where \begin{align*}s\end{align*} and \begin{align*}s^\prime\end{align*} are the respective object and image distances, and \begin{align*}f\end{align*} is the focal length. An easy way to determine whether a mirror is concave or convex or if a real or a virtual image is produced is to look at the sign of the variables in the above equation.

  • Using the following table, you will be able to tell what type of image you will see before looking into a mirror.


+ if the object is in front of the mirror (real)

- if the object is behind the mirror (virtual)


+ if the object is in front of the mirror (real)

- if the object is behind the mirror (virtual)


+ the curvature is in front of the mirror (concave)

- the curvature is behind the mirror (convex)

Show What You Know

  1. What is the minimum number of rays needed to locate an image that is formed by a mirror?
  2. If the focal length approaches infinity for a plane mirror, what relationship exists between the object location and the image location?
  3. If the focal length of a convex mirror is -5 cm, and an object is placed 10 cm from the mirror, where is the image formed?

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Image Attributions

  1. [1]^ Credit: Flickr: Eden, Janine and Jim; Source: http://www.flickr.com/photos/edenpictures/2209366634/; License: CC BY-NC 3.0
  2. [2]^ Credit: Flickr: SBA73; Source: http://www.flickr.com/photos/7455207@N05/4463257694; License: CC BY-NC 3.0

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