Light and Optics Light and Optics Physics Help Forum Jan 28th 2015, 07:46 PM #1 Junior Member   Join Date: Jan 2015 Posts: 1 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!  Tags lens, optics Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post siddhartha5432 Light and Optics 1 Oct 23rd 2013 09:53 AM dank420 Light and Optics 4 Sep 9th 2009 10:29 PM songoku Light and Optics 2 Sep 8th 2009 04:50 AM NAKA Advanced Optics 0 Nov 11th 2008 05:20 PM NAKA Advanced Optics 0 Nov 11th 2008 05:16 PM