Why concave lens is used in peephole?

Your answer is similar to CE Physics 2002 Paper 1 Q.10 (b)

The concave lens cannot be replaced by a concave lens because
* the image formed may be inverted.
* the image formed may be magnified and it is difficult for Kitty to identify the visitor through the peephole.
* the image formed may be real and it is difficult for Kitty to observe the image.
* the field of view of the peephole becomes narrower.

Anyone of the above.

The first reason is the easiest one to understand. If you use a convex lens, the image formed may be inverted. You may consider the case that the object distance is greater than or equal to focal length. An inverted image of course difficult to observe. But, if you use a concave lens, the image formed must be erect.

Therefore, a concave lens is chosen for peephole.

2007-10-16 03:02:28 補充：
The first sentence should be "Your 'question' is similar to CE Physics 2002 Paper 1 Q.10 (b)" Sorry for the typo.

2007-10-16 11:03:47 補充：
"The concave lens cannot be replaced by a concave lens because" should be "The concave lens cannot be replaced by a 'convex' lens".Again, sorry for the typo.

To have a wider field of view.