Question�
    
I have a thin piece of plastic mounted on the back window of my RV. It 
magnifies things so I can see better when I am backing up. Why can such a 
thin piece of plastic magnify things? A normal glass magnifying lens would 
have to be curved on both sides and would be much thicker. Answer
    

If you have ever looked at the lens of a magnifying glass, you know it is 
thick in the middle and tapers to nothing at the edges. In other words it is 
shaped like a lentil, which is where the word lens comes from. It would not 
be very easy to make a big magnifying glass lens for your RV because it would 
be thick, heavy and hard to mount. The thin piece of plastic you are using is 
called a Fresnel lens. It is flat on one side and ridged on the other. 
Fresnel lenses we first used in the 1800's as the lens that focuses the beam 
in lighthouse lamps. Plastic Fresnel lenses are used as magnifiers when a 
thin, light lens is needed. The quality of the image is not nearly as good as 
that from a continuous glass lens, but in lots of applications (like your RV) 
perfect image quality is not necessary. The basic idea behind a Fresnel lens 
is simple. Imagine taking a plastic magnifying glass lens and slicing it into 
a hundred concentric rings (like the rings of a tree). Each ring is slightly 
thinner than the next and focuses the light toward the center. Now take each 
ring, modify it so it flat on one side, and make it the same thickness as the 
others. To retain the rings' ability to focus the light toward the center, 
the angle of each ring's angled face will be different. Now if you stack all 
the rings back together, you have a Fresnel lens. You can make the lens 
extremely large if you like. Large Fresnel lenses are often used as solar 
concentrators.