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Thread: Trainee Dispensing optician (UK) Help !!

  1. #1
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    Trainee Dispensing optician (UK) Help !!

    Hi,
    I am Nicola in the UK.
    Stuck on a few questions.
    Not looking to be spoon fed but some help would be great.
    Here goes
    A thin plus lens, made in glass of refractive index 1.50, has a focal length of +200mm and a concave surface radius of +125mm. What is the radius of the convex surface ?

    If some one could talk me through it that would be great. :cheers:

  2. #2
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by NicolaKirk View Post
    Hi,
    I am Nicola in the UK.
    Stuck on a few questions.
    Not looking to be spoon fed but some help would be great.
    Here goes
    A thin plus lens, made in glass of refractive index 1.50, has a focal length of +200mm and a concave surface radius of +125mm. What is the radius of the convex surface ?

    If some one could talk me through it that would be great. :cheers:
    Focal length of +200mm, convert that to diopters:

    D = 1/f
    D = 1/0.2m
    D = +5.00

    Concave radius of 125mm, convert that to diopters:

    F2 = (n - 1) / R
    F2 = (1.5 - 1) / 0.125m
    F2 = 4.00D and concave means (-)

    Now our lensmakers equation:

    D = F1 + F2
    5.00 = F1 + (-4.00)
    F1 = 5.00 - (-4.00)
    F1 = 9.00

    Now run this back through the above equation to convert to radius.

    The goal is to convert these measures into diopters a measure that we can add and subract from each other. Once that is done the lensmakers equation allows us to add and subtract them. Since they are thin plus lenses the directions are telling us to ignore the thickness.

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