prisms as perfect mirrors

When the light passes from air throguh the glass prism,if the angle of incident is greater than 42,which is the critical angle of the glass prism,total internal reflection occurs.When the angle is smaller than 42,there are both refracted and reflected light,so there are also mutiple images.

圖片參考：http://hk.geocities.com/viceleader982040/23132141.gif

For ordinary plane mirror,there is a silvery coating on the mirror(slightly diffrent on the above diagram).When the light from the ball passes through the glass,it is refracted as shown above,in other words,the light is refracted by the silvery coating and thus form mutiple images.However,the formation of mutiple images by prism is better because the light is not refracted by the silver,this is the only possible reason

2008-06-28 23:09:06 補充：
There is no silvery coating on the prism!

The angle 42 degree is named the "Angle of Total reflection" of that glass prism.

Basically, when a light beam is incident on a surface (from prism to air), then it can either be reflected only or both reflected and refracted.

The "Angle of Total reflection" tells you how big the incident angle should be in order to have 100% reflection and 0& refraction. That is the idea to keep light beam in a glass medium (Optical Fiber)

So, if the incident angle is smaller than 42, it can be a less efficient mirror (because of its ability to reflect) and you can see an image. If you allow the light to relflect more than a time inside the prism, then ofcourse you would receive more than an image, because whenever there is a reflection occur, there is also a refraction and those refracted from the prism can be received by your image detector.

2008-06-26 17:10:43 補充：
I do not understand how the multiple images come from, would you draw some picture? Especially for an ordinary mirror, how can one mirror generate multiple image?
Thanks!