help

[lee staniforth]

A +8.00D lens is to be made in glass of refractive index 1.701 using tools calibrated for spectacles crown glass of refractive index 1.523. A -3.00D tool is chosen to work the back surface. If the axial thickness of the lens is 6.5mm, which tool must be used for the front surface?

The answer booklet says its +8.57 but i cannot determine how they arrived at that figure.

thanks anybody who can help

[Darryl Meister]

You will need to work with a formula known as the curve variation factor to determine the actual refractive power of the surface when the specified surface power is based upon a theoretical refractive index or "tooling" index that differs from the actual refractive index of the material. To convert "tool" surface power to the actual refractive power of the surface, use:

Or,



Next, you will need to determine the necessary front curve by rearranging the back vertex power formula:


which, after some lengthy rearranging to solve for Front yields,

Or,



Which is the actual refractive power of the front surface necessary to produce the desired back vertex power of +8.00 D. Finally, you must convert this refractive surface power back into "tool" power by rearranging the curve variation factor shown above:

[Darryl Meister]

PS,
You will need to ensure that your OptiBoard background color is not set to black in order to see the equations.