It is important, it is very important, that we recognize and emphasize the difference between plus and minus lenses.
|Plus is like a hill||Minus is like a hollow|
|Plus stands for positive||Minus stands for negative|
|Plus magnifies||Minus reduces|
|Plus means convex||Minus means concave|
|Plus adds curvature||Minus subtracts curvature|
|Plus helps nearer seeing||Minus helps farther seeing|
The lenses in opera glasses, camera lenses, magnifiers and microscopes are all focus lenses. They all have plus (convex) curvatures which bend or refract light and heat rays which fall on their surfaces. They bend the rays towards an imaginary line from the optical center of the lens to the focus, which is some distance from the lens. This distance depends on the curvature of the lens. A little curvature will have the focus far away. Mor curvature will bring the focus nearer.
Colored glasses and goggles are not focus-glasses because they are either flat or neutral. Prism lenses are not focus lenses because they bend light rays into a line instead of a point or focus. Focus lenses are those which are either plus or minus.
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