Types of surfaces and lenses. 
Characteristics of plus and minus lenses.
PAGE References to Optical Formulas Tutorial: (first reference is to edition 1 / second reference is to edition 2).

Characteristics of Plus and Minus Lenses

For this lesson you need a spherical plus lens and a spherical minus lens.

Start with the plus lens. Place the lens between your thumb and forefinger and run your fingers from the center to the edge. You can tell that it is thicker in the center than at the edge.

Look through the lens, holding it up close to your eye. Everything looks bigger through it, and right-side up. Hold it out at normal reading distance and look through it at something that is an inch or two on the other side of it. The object that you are looking at will appear to be bigger than normal, and right-side up. Still holding it away at normal reading distance, look through it at something that is several feet away. The image will be up-side down. Images that are upside-down are real images. Images that are right-side-up are virtual images. Sometimes the plus lens has a virtual image, sometimes it has a real image.

Still holding the lens away at normal reading distance, look through it at the edge of a paper or a book that is only a few inches further away. Hold it so that it is completely over the paper, but at the edge of the paper. You will not see the edge of the paper through the lens. Now, move the lens slowly over the edge of the paper, and watch what happens to the IMAGE of the edge of the paper. At first the image of the edge of the paper is outside of the real edge. As you slowly move the lens sideways, the image of the edge will approach the real edge, until the middle of the lens is lined up with the edge of the paper; then the image will be where it is supposed to be. As you continue moving the lens sideways, the image of the edge will continue to move in the opposite direction.

Lenses are rarely used alone. They are almost always a part of an optical system comprised of several refracting surfaces. The eye itself is an optical system comprised of four major refracting surfaces. When you place a lens in a frame in front of an eye, the lens becomes a part of the optical system. In any optical system, the distance between the refracting surfaces of the system affects the power of the optical system.

If I have two lenses together, one a minus lens and one a plus lens with equal but opposite powers, their powers cancel each other or neutralize each other. If I move the lenses apart they no longer exactly neutralize each other.

A plus lens moved away from the optical system gains more plus power with respect to the optical system. This is an increase in effective power. The lens by itself did not gain or loose power; but its effect on the optical system gained plus power. If the plus lens is moved toward the optical system it looses effective plus power.
 

CHARACTERISTICS OF A PLUS LENS
     
  1. Converges parallel incident light rays.
  2. Has plus power.
  3. Thickest in the center, thinnest at the edges.
  4. Can have a real image.
  5. Can magnify images.
  6. Shows against motion.
  7. Increases in effective power as it moves away from the eye.

Now put the plus lens aside and look at the minus lens. Repeat all of the suggestions that I made for the plus lens.

You can feel that it is thinnest at the center, and thickest at the edges.

Regardless of where either your eye or the object that you are looking at is with respect to the lens, the image is right-side up and smaller than it really is. Since the image is always right-side-up, it is always virtual. We call an image that is bigger than the object magnified, as I'm sure you already know. We call a smaller image minified. I'll bet you did not know 'minified' was an acceptable word. You will not find it in Webster's. It is, however, an acceptable word for us.

When you look at the edge of the paper through this lens, with it held completely over the paper but at the edge of it, you can already see the edge of the paper through the lens. As you move the lens sideways the image of the edge moves in the same direction as the movement of the lens. It still lines up correctly at the center, then keeps moving with the lens as you continue to move the lens sideways.

As the minus lens is moved away from an optical system it loses minus power (which is the same thing as saying that it 'gains plus power').
 

CHARACTERISTICS OF A MINUS LENS
     
  1. Diverges parallel incident light rays.
  2. Has minus power.
  3. Thickest at the edge, thinnest in the center.
  4. Has only virtual images.
  5. Minifies images.
  6. Shows with motion.
  7. Decreases in effective power as it moves away from the eye.

Please read pages (31-33 / 47) in Optical Formulas Tutorial.   You will be responsible for theses lens types.  After reading those pages, note these rules of thumb:
 
 

TYPES OF SURFACES AND LENSES:
  1. CONCAVE/CONVEX
    • If the surface is not curved it is plano, abbreviated pl.  It has no or 0 power.
    • If the surface bulges toward the observer it is convex.  We consider this to be a plus or positive surface.
    • If the surface is hollow or cave-like it is concave.  We consider this to be a minus or negative surface.
  2. FLAT/BENT:
    • If one surface is concave and one surface is convex, it is a bent lens.  Meniscus, which is related somehow to the crescent moon, is a generic name for a bent lens.  A meniscus lens can have no power, plus power, or minus power.  [Mene is Greek for moon or a phase of it;  So meniscus, according to Webster's, is a crescent-shaped thing.]
    • If the lens is not bent it is flat.  Flat does not mean that one surface is plano.   It can have no power, plus power, or minus power.
    • Lens, by the way, is a transparent material with two polished surfaces, at least one of which is curved.  Is a piece of window glass a lens? 

 


 
 
 

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