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The Lensmakers Equation as written in this Ontario Georgian College video production uses measurement units
in meters (m), rather than centimeters (cm) that this instructor prefers. The Lensmakers Equation used in this video… 1/l + 1/f = 1/l’
is valid only if the units of measurements used for l= Object distance and l’ = Image distance
is meters (m), rather than centimeters (cm).
Vergence Formula:
L + F = L’
The object ray here has an incoming minus or negative vergence
L in Diopters (D), strikes
F, a +4.00lens, and then exits with plus or converging vergence
L’ in D. to the right of the lens. The inverse or reciprocal of incoming L vergence is l or object distance, lens F reciprocal is focal length, and the reciprocal of exiting vergence of image L’ is image distance. Since L = 1/l(m) F=1/f(m) and L’ = 1/l’(m)
L + F = L’ becomes
1/l(m) + 1/f(m) = 1/l’(m) or Lensmakers Equation as used in this Georgian College video.
If this Georgian College opticianry instructor choose to work in centimeters (cm) measurements rather than meters (m), her Lensmakers Equation must use 100 (the number of centimeters in a meter) rather than 1 (in meters) in the numerator of her Lensmakers Equation. Her Lensmakers Equation on the whiteboard should have read
100/l(cm) +100/f(cm) = 100/l’(cm) or
100/-60cm + (100/25cm) = 100/l’cm
-1.67D + (+4.00D) = 100/l’cm
+2.33D = 100/l’cm
Thus image distance l’cm = 100/+2.33D = +42.918cm to right of lens.
Georgian College head opticianry instructor Janice Schmidt made this video while teaching Georgian College’s 2-year opticianry program. Yet at 12:12 of her 19-minute video, she simply plugs in centimeters and placed no units of measurements in her Lensmakers Equation. If her other Georgian opticianry instructors watched this Georgian College video, none realized her glaring error.
She used 1/l + 1/f =1/l’ without specifying units of measurements, and then simply pluged in 60 cm for object distance, 25cm for focal length and attempted to solve for image distance l’ in centimeters.
1/60 +1/25 = 1/l’
which is incorrect, and simply cannot result in the 42.91cm image distance that she claimed.
So much for Georgian College’s vaunted reputation as the best opticianry program in Canada. I have to wonder if any of her students challenged her result in this video. Vergence is one of the trickiest areas of an opticianry curriculum, especially for thick lenses. I even had one BC College of Optics student say that he wanted to design a T-Shirt that screamed …
“I SURVIVED VERGENCE”
So if you can, I suggest that you travel to BC and register for our next 6-month BC College of Optics OPTICIAN/CONTACT LENS FITTER program.
Next classes will begin September 19th, 2016.
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