Results 1 to 3 of 3

Thread: how do i solve this?

  1. #1
    OptiBoard Novice
    Join Date
    Jan 2019
    Location
    London, United kingdom
    Occupation
    Dispensing Optician
    Posts
    8

    Confused how do i solve this?

    Hi

    calculate the back vertex power of a lens made from glass n = 1.5. with an axial thickness of 9mm if the front surface has a power of +10.76D and the back surface has a power of -3.00D?

    so far I have:

    n= 1.5
    t= 9mm
    F1= +10.76D
    F2= -3.00D
    F= +7.76D

    I used this formula:
    t/n which is 9/1.5 = 6mm

    if someone could point out some guidance, that would be great. thanks

  2. #2
    Master OptiBoarder OptiBoard Silver Supporter
    Join Date
    Jul 2002
    Location
    Vancouver, BC CANADA
    Occupation
    Dispensing Optician
    Posts
    1,106
    I suggest you purchase Ellen Stoner's excellent book 'Optical Formulas'.

  3. #3
    Master OptiBoarder MakeOptics's Avatar
    Join Date
    Aug 2011
    Location
    none
    Occupation
    Other Eyecare-Related Field
    Posts
    1,327
    Quote Originally Posted by londongirl312 View Post
    Hi

    calculate the back vertex power of a lens made from glass n = 1.5. with an axial thickness of 9mm if the front surface has a power of +10.76D and the back surface has a power of -3.00D?

    so far I have:

    n= 1.5
    t= 9mm
    F1= +10.76D
    F2= -3.00D
    F= +7.76D

    I used this formula:
    t/n which is 9/1.5 = 6mm

    if someone could point out some guidance, that would be great. thanks
    Start by removing the thickness from your equation and creating a thin lens equation, to do this convert the front surface to the effective power at the back surface.

    Convert front to focal distance:

    1.5 / +10.76D = 0.1394m

    Now since you are traveling 9mm through the lens remove that:

    0.1394m - 0.009m = 0.1304m

    Now convert back to diopters:

    1.5 / 0.1304m = +11.50D

    Now you have a thin lens equivalent so just plug that into the thin lens equation:

    +11.50D + (-3.00D) = +8.50D


    The formula you are using is converting the thickness to the air equivalent so 9mm / 1.5 = 6mm, which now if you used would work the same as the steps above but instead of using the index of the material to convert the front surface you would use the index of air since you compensated out the thickness:

    1 / +10.76D = 0.0929m

    Subtract the air equivalent thickness:

    0.0929m - 0.006m = 0.0869m

    Now convert back to diopters:

    1 / 0.0869m = +11.50D

    Now plug that into the thin lens equation:

    +11.50 + (-3.00D) = +8.50D

    For more information look up Swiane's step along method, and vergence formulas to gain a better understanding. Or you could just use a formula which I believe in the book mentioned above is an approximation.
    http://www.opticians.cc

    Creator of the industries 1st HTML5 Browser based tracer software.
    Creator of the industries 1st Mac tracer software.
    Creator of the industries 1st Linux tracer software.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Light streaking through new spectacles. Even company can't solve. Please help.
    By debayandas in forum General Optics and Eyecare Discussion Forum
    Replies: 71
    Last Post: 07-11-2017, 03:14 PM
  2. HELP me solve this puzzle- CURVES?
    By misstara007 in forum General Optics and Eyecare Discussion Forum
    Replies: 23
    Last Post: 04-19-2009, 02:49 AM
  3. Solve this problem!!
    By soumya in forum General Optics and Eyecare Discussion Forum
    Replies: 25
    Last Post: 10-04-2007, 10:55 PM
  4. Help solve a Murder Mystery!
    By Steve Machol in forum Just Conversation
    Replies: 5
    Last Post: 06-25-2001, 08:46 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
OptiBoard is proudly sponsored by:
Younger Optics and Vision Equipment