A technician wishes to produce the lens +2.00DS/+2.00DC x90 in toric form, in CR39 (n=1.498) with a -5.00 base curve. After finishing the toroidal surface with the CR39 tool it is discovered that, in error, they have used a plastics material of refractive index 1.56. Which CR39 tool must now be used to work the front surface?

Ok i will try to look it up on the internet and see if i can find the answer.

Oh, bless ya. I have tried doing the question but don't know what to do. I tried transposing the prescrition first to +4.00DS/-2.00DC x 180 then not sure what to do

I did it your way and got -360. Did you get that? I was just wondering. I have figured it out yet.

Nope, how did u get -360? I'm stuck not sure what to do next

Yeah..............Now I'm confused. :(

Never mind, don't think anyone has the answer to this!lol Thanks for trying though

Your welcome. Sorry i couldn't help you.

But this is a good website. MS. Sue is good a English so is writeatecher. Steve is good at math.

Great, thank you :)

To determine the correct CR39 tool to be used after the error, we need to calculate the new base curve for the front surface of the lens.

First, let's understand the situation. The technician mistakenly used a plastics material with a refractive index of 1.56 instead of the desired CR39 material with a refractive index of 1.498. Using the wrong material will result in a change to the overall thickness and curvature of the lens.

The prescription of the lens is +2.00DS/+2.00DC x90. The back surface has already been finished with a -5.00 base curve. Now we need to calculate the new base curve for the front surface.

To calculate the new base curve, we can use the lens formula:

1/f = (n2 - n1) * (1/R1 - 1/R2)

Where:
f is the focal length of the lens
n1 is the refractive index of the initial material (1.56 in this case)
n2 is the refractive index of the desired CR39 material (1.498)
R1 is the radius of curvature of the back surface (which is -1/(-5.00) = 0.2)
R2 is the radius of curvature of the front surface (which we want to find)

Substituting the values into the lens formula:

1/f = (1.498 - 1.56) * (1/0.2 - 1/R2)

Since the lens is plano in the front (+2.00DS), the focal length is infinite (f = 1/infinity = 0). Therefore, we can simplify the equation:

0 = (1.498 - 1.56) * (1/0.2 - 1/R2)

Simplifying further:

0 = -0.062 * (5 - 1/R2)

0 = 0.248 - 0.062/R2

0.062/R2 = 0.248

R2 = 0.062/0.248

R2 = 0.25

The radius of curvature (R2) of the front surface should be 0.25 when using the correct CR39 material.

Therefore, the CR39 tool with a base curve of -0.25 should now be used to work the front surface of the lens.