Extra Questions for
Module 5 -- Focal length formula, Diopters.


Each worksheet is different, because each one was originally written by a different person.  Use the directions at the beginning of each worksheet for that worksheet.
 

Focal Power Formula

D = 1/f, where f is in meters;  or D = 39.37/f, where f is in inches.  (1 meter  = 39.37 inches.)
 

  1. Find the focal lengths of the given lenses:
      2.  
    1. D = +2.00D. f = ______ meters.
    2. D = +10.00D.  f = ______mm.
    3. D = -0.50D,  f = ______ cm.
    4. D = -2.50D,  f = ______ inches.
    5. D = +2.12D,   f = _____ mm.

      6.  

       
       
       

  2. Find the Dioptric power of lenses having the following focal lengths:
        3.  
      1. f = +4.0 meters, D = ______D.
      2. f = +20 mm, D = ______ D.
      3. f = -39.37 inches, D = ________ D.
      4. f  = -25 cm, D = ________ D.
      5. f = +4.93 inches, D = ______ D.
WORKSHEET
FOCAL LENGTH FORMULA

D = 1/f

  1. What is the dioptric power of a lens with a focal length of
    1. 25 mm
    2. 2.5 mm
    3. 25 ft
    4. 25 in
    5. 25 cm

      6.  
  2. What is the focal length of a +5.00 D lens in:
    1. meters?
    2. mm?
    3. inches?

      4.  
  3. What is the focal length of a +0.50 D lens in
    1. feet ?
    2. cm?
    3. meters?

      4.  
  4. What dioptric power is required to focus at 40 cm?
Focal Length Worksheet
  1. What is the focal length of a +3.00 D lens?

    2.  
  2. What is the power of a lens if it brings incident light to a focus 20 mm away?

    3.  
  3. What is the focal length of a -0.50D lens?

    4.  
  4. What is the focal length of a -2.50D lens?

    5.  
  5. What is the power of a lens with a virtual focus 20 mm away?

    6.  
  6. What is the power of a lens with a focus 4.92 inches away?

    7.  
  7. What is the focal length of a +10.00D lens in cm?

    8.  
  8. What is the focal length of a +10.00D lens in inches?

    9.  
  9. A +2.12D lens will focus light ______ mm away.

    10.  
  10. A lens focuses light 4m away.  What is the power of the lens?

    11.  

     

  11. A minus lens has a focal length of 25 cm.  What power is the lens?

    12.  
  12. A lens with a focal length of 79.5 inches is a _____ D lens.

The answers are here.  Please do them yourself before you go to the answers.  If you understand it all and you don't need the practice then it will only take you a few minutes to do them.  If you are struggling with the formulas do one or two, check them, do a few more, check them, . . . just make sure that you do not look ahead of the ones that you did and are checking.

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