Objective:
    - to learn about lenses and their applications

Applications:
    - lenses are used in such fields as photography, optometry, and many fields of science (such as biology and astronomy)

Concepts:
    - 2 types of lenses:
convex, which curves out and converges light rays
    (creates a real, or upside down, image) - has a positive focal point
concave, which curves in and diverges light rays
    (creates a virtual, or right side up, image) - has a negative focal point
- lenses can be used for the creation of such instruments as camera lenses, eye glasses, or microscopes and telescopes (to varying degrees of power), for practical, functional, or scientific use(s)

Formulae:
    1/f - 1/do = 1/di    or    (f^-1) - (do^-1) = (di^-1)
f = the focal point (the specified distance from the center of the lens, on
    both sides)
do = the distance to the object, from the lens (always positive)
di = the distance to the image (projected), from the lens (always positive)

    di/do    or    dido^-1
either will solve for the magnification of the image (projected) from the
    object (original)

Exempli:
- the first light ray from the object is referred to as the initial ray one, or "ir1" (the second is referred to as initail ray two, or "ir2")
- reflected rays are corresponding to their initial ray, or as "rr1" and "rr2"
- a virtual ray, "vr1" or "vr2", is a non existant ray, but is used to determine where the image appears

If an object is 15 centimeters away from a convex lens with a focal point of 5 centimeters, what is the distance to the image (di)?

1/f - 1/do = 1/di = 1/5cm - 1/15cm = 2/15(cm)

If an object is 15 centimeters away from a concave lens with a focal piont of 5 centimeters, what is the distance to the object (di)?

(f^-1) - (do^-1) = di^-1 = -(5cm^-1) - (15cm^-1) = -4/15(cm)

In a double lens applicaiton, the focal length of lens one must be greater than the focal length of lens two.  The focal length of lens one minus the focal length of lens two must be less than the focal length of lens two.  Also, the secondary "di" will be a virtual image.

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