Get a little help here, please?

I am reading a book called "Optics" by M.H. Freeman (10th ed) and am
stumped by two of the problems in ch. 4 "Thin Lenses":

24. A thin converging lens of focal length f is used to form an image
of a luminous object on a screen. Determine the least possible
distance between object and screen.
Answer (from answer key): 67.08 cm

On this one, I just don't see how you can get to the answer without
knowing the value of f. And if the question means for any value of f,
then I would think the answer would be 0?


26. A thin biconvex lens having curvatures of 3 and 5 D and n = 1.62
is immersed in a large tank of water. An object is placed in the
water 1 m from the lens; what will be the position of the image if
that is also formed in the water?
Answer: 138.6 cm from lens

On this one, the solution seems straightforward, but the answer I get
doesn't match the book's answer.
Taking the refractive index of water = 1.334, I calculate the power at
the two surfaces to be .858 and 1.43, using the formula (n' - n) * R.
Then the power of the lens is 2.288, that is, F = F1 + F2.
Then applying the formula 1/l' = 1/l + 1/f', as follows:

1/l' = 1/-1 + 2.288
-----> 1/l' = 1.288
-----> l' = .776 m, or 77.6 cm.

Any help would be appreciated. I am doing this on my own time and not
for a class.

.

Relevant Pages

  • Re: AirOptix for Astigmatism?
    ... Interesting that since the first si-hy lens came out, N&D, several years ago ... Wouldn't the more oxygen permeability=better for the eyes? ... create lenses with higher dk/t than N&D. ... higher water content, actually have a lower water content--like Oasys, Air ...
    (sci.med.vision)
  • Re: PureVision Toric lenses
    ... My new optometrist just tried a couple different Biomedic toric lenses on ... them yet, but was wondering what opinions you eye docs have of these lenses, ... how does a lens' water content affect tear levels in theeye? ...
    (sci.med.vision)
  • Re: Eyepiece cleaning question after Katrina
    ... As long as the lens are not glued together, the best way to get rid of the ... salts and crud is to use luck warm water, ... water offen or a in a basket under running water. ... clean and lub the iris & barrle movment areas with the right lub. ...
    (sci.astro.amateur)
  • Re: Groundwater lens?
    ... >> What you define would not be considered a lens. ... >> contained in it can be considered a lens of water. ... I believe perched water tables are impossible from the OP's ... >> It's possible only in a very arid region where an intermittent river is ...
    (sci.geo.geology)
  • Re: Extreme H2O contact lenses for dry eyes
    ... This issue of contact lens comfort and dry eye comes up often in this ... It is true, however, that some lower water content ... It turns out that brand A might be fine to use with one ...
    (sci.med.vision)