Equations used with lenses

It is important to remember your signs when using these equations.  An object which is to the left of the lens will have a positive sign.  An image to the right of the lens will also carry a positive sign. 

·        Lens Equation:

1/f = 1/d_i + 1/d_o

·        Magnification equation:

The magnification of an object is equal to the ratio between the size of the image and the size of the object.

s_i/s_o = -d_i/d_o

Example:

            A convex lens has a focal length of 30cm.  A glass figurine that is 2cm tall, is placed 40cm from the lens.  Where is the image located, and what is the magnification, and what is the size of the image formed?

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