Re: measuring lens index of refraction & dispersion

I got some non-credible values for n (index of refraction) on my three
lenses.

My method and info follows. I would appreciate any tips on what may be
wrong.
I double-checked the spread*** so I think errors are in my
interpretation of conventions rather than the math.

No spherometer yet, but based on telescope mirror makers practices, I
used a 5-digit (x.yyyy") machinist's vertical depth indicator with a
fine point round tip on it's own reference plate base and a 4-digit
digital caliper. Both are periodically calibrated.

I measured the diameters, overall height, edge thicknesses and depth
of concave surfaces, finding center (deepest part) by repeated
measurements. I understand some error may exist there. The meniscus
lens has a 0.002" discrepancy between measured and subtracted
calculation on the concave side. I understand ROC on the flat side of
the PCV is infinite. I think my intent is clear on the sketches (and
may reveal an error).

I calculated the convex curvatures by subtracting edge thicknesses
from overall height etc.

I calculated ROC and N from the ROC values. N values are too high, at
least only tens of percentage points off (better than an order of
magnitude). I went back and changed sign of ROC's for opposite facing
curves (only slight difference resulted).

Possible sources of error:

1) Method of measurement: Is there any reason this measurement method
would be any less valid than a spherometer?

2) The declared focal lengths and diameters came from the place I
bought them. The measured diameters showed some of the specs were
accurate and some were 'nominal'. I am beginning to wonder if the f.l.
specs are reliable. I can ask him (Fred at Surplus Shed) whether they
came with specs or are his measurements.

3) I made some error in convention in my determination of the
magnitude and direction of the sagittae.

4) I used the thin lens formula to calculate n. I didn't check yet to
see if n can be solved from the ROC's alone with the full form of the
lensmaker's equation.

Thank you.

Murray


Sketches with measured and calculated dimensions:
http://www.uptowngallery.org/Murray/spherometry/lens_forms.jpg

Equation:
http://www.uptowngallery.org/Murray/spherometry/thin_lens.jpg

Spread*** output:
http://www.uptowngallery.org/Murray/spherometry/calculations.jpg

Spread***:
http://www.uptowngallery.org/Murray/spherometry/SSglass.xls

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