Concave Mirror

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Concave Mirror

• The center C of a concave mirror is outside the
  mirror.
• Focal point F is also outside the mirror, half
  way between the center and the surface of the
  mirror.
• The focal length f is half of the radius.

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Significance of the focal point

• All light rays that are parallel to the axis will
  pass (after extrapolating for convex mirror)
  through the focal point!
• For a concave mirror, the light can pass through
  the focal point.

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Image from a concave mirror case 1
Step 1
Step 2
Step 3
Step 4
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Image from a concave mirror case 1
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Properties of the Image

• If we put an object outside of the center of a
  concave mirror, we find the image is
• Real, in the sense that all light rays pass
  through the image.
• Inverted, in the sense that the direction of the
  arrow has been changed.
• The image is smaller!

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Animation for case 1
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Image for a concave mirror case 2
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• If the object is in between the center and the
  focal point, the image is
• Real
• Inverted
• Magnified in the sense that the image is bigger
  than the real object.
• The direction of the light rays can be reversed
  and the result is still physically possible! (or
  the image and object interchanged)

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Animation for case 2
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Image from a concave mirror case 3
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Image from a concave mirror case 3
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Animation for case 3
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Properties of the image

• If the object is closer to the mirror than the
  focal point F, the image is
• Virtual, it is behind the mirror
• Upright, not inverted
• Magnified
• Can be used for shaving!

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Summary
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Java Applet

• http//www.microscopy.fsu.edu/primer/java/mirrors/
  concave.html

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Some of the really-cool applications

• Suppose you put a point source of light at F. All
  rays will be reflected back parallel to the axis
  of mirror a neat way to construct a parallel
  beam!
• Applications flashlight! Headlight in the car.
• Conversely, if one has parallel rays, all are
  reflected to pass through F. So all light energy
  is focused to one point!