物理凹透鏡

圖片參考：http://www.tutornotes.com.hk/wp-content/uploads/2011/06/concave_lens_2-300x187.jpg

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concave lens

圖片參考：http://imgcld.yimg.com/8/n/HA07276071/o/701203170023413873449790.jpg

For parallel light rays that are parallel to the principal axis, after passing through a concave lens (凹透鏡), the light rays will diverge. On extending the divergent rays backward, they will meet at the focus(焦點) of the lens.

For parallel light rays that are not parallel to the principal axis, after passing through the concave lens, the divergent light rays, when extending backward, will meet at a point on the focal plane (i.e. at points above or below the focus).


圖片參考：http://imgcld.yimg.com/8/n/HA00458726/o/701203170023413873449780.jpg

In the above diagram, the black thin lines are construction lines. The red lines are refracted light rays.


2012-03-17 19:13:28 補充：
You may look at the picture by the link below if the pitcure doesn't show out in the answer.

http://i250.photobucket.com/albums/gg258/easygophoto/show2/concavelens.png