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Old Jan 28th 2015, 07:46 PM   #1
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Join Date: Jan 2015
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Lens Help?

Hello,

Thanks so much for reading this. I think I'm close I"m just not confidant in my answer. The problem reads: For a thin plano-convex lens, what is the change in focal length (Delta f) for a 100 um change in radius (delta R) of the convex surface. Assume the lens has a visible-wavelength refractive index of n=1.5.

I know 1/f=(n-1)(1/R1-1/R2)

For the plano side the R1 is going to be infinite I think. Which gives me n and R1. Can I do the equation twice, once with R2=100 and once where R2=200, solve for f in each case, and then subtract the results? i.e.

1/f1=(n-1)(1/R1-1/R2)
1/f1=(1.5-1)(1/Infinity-1/100)
f1=-200

1/f2=(n-1)(1/R1-1/R2)
1/f2=(1.5-1)(1/Infinity-1/200)
f2=-400

delta f = f1 - f2
delta f = 200

Thank you again for your feedback!
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