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 = +0.50 meters.
    2. D = +10.00D.  f =  +100  mm.
    3. D = -0.50D,  f =  -200  cm.
    4. D = -2.50D,  f = -15.748 inches.
    5. D = +2.12D,   f =  +472  mm.

      6.  

  2. Find the Dioptric power of lenses having the following focal lengths:
        3.  
      1. f = +4.0 meters, D =  +0.25 D.
      2. f = +20 mm, D =  +50  D.
      3. f = -39.37 inches, D =  -1.00  D.
      4. f  = -25 cm, D = -4.00 D.
      5. f = +4.93 inches, D =  +7.99  D.  (or  +8.00D )
WORKSHEET
FOCAL LENGTH FORMULA

D = 1/f   (or   D = 40/f, in inches)

  1. What is the dioptric power of a lens with a focal length of
    1. 25 mm    40
    2. 2.5 mm    400
    3. 25 ft  = 300 inches  0.1333
    4. 25 in    1.6
    5. 25 cm    4

      6.  
  2. What is the focal length of a +5.00 D lens in:
    1. meters?    0.2
    2. mm?    200
    3. inches?    8

      4.  
  3. What is the focal length of a +0.50 D lens in
    1. feet ?   80 inches =  6.67 ft
    2. cm?    200 cm
    3. meters?    2 m

      4.  
  4. What dioptric power is required to focus at 40 cm?    2.5 D
Focal Length Worksheet
  1. What is the focal length in meters of a +3.00 D lens?    +0.333 m

    2.  
  2. What is the power of a lens if it brings incident light to a focus 20 mm away?    +50D

    3.  
  3. What is the focal length (in meters) of a -0.50D lens?    -2 meters

    4.  
  4. What is the focal length in cm of a -2.50D lens?    -0.4 meters or -40 cm or -400 mm . . . 

    5.  
  5. What is the power of a lens with a virtual focus 20 mm away?    -50D

    6.  
  6. What is the power of a lens with a focus 4.92 inches away?    +8.13D if you use D=40/f,  or +8.00 if you convert using 1 m = 39.37 inches

    7.  
  7. What is the focal length of a +10.00D lens in cm?    +10 cm

    8.  
  8. What is the focal length of a +10.00D lens in inches?    +4

    9.  
  9. A +2.12D lens will focus light ______ mm away.    +472 mm

    10.  
  10. A lens focuses light 4m away.  What is the power of the lens?    +0.25 D
  11. A minus lens has a focal length of 25 cm.  What power is the lens?    -4

    12.  
  12. A lens with a focal length of 80 inches is a _____ D lens.    +0.50 D

 


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