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Thread: Ray tracing question: I am at my wit's end!!

  1. #1
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    Angry Ray tracing question: I am at my wit's end!!

    I am studying for the abo ac and right now I'm so mad I could spit.
    Question: When the path of light is drawn through a lens, the primary focal point of a diverging lens is:

    the answer is "to the right of the lens".
    How the &**(#@ is this figured out? There are other questions similar to this one. I thought it was a prism type thing with rays going towards the base but it's not.
    I have no clue where to look for this. I am studying the Optical Math Tutorial.

    Can someone please help me? I am trying to get help from different sources and no one is biting.

    Thanks

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    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by guinea_guinea View Post
    I am studying for the abo ac and right now I'm so mad I could spit.
    Question: When the path of light is drawn through a lens, the primary focal point of a diverging lens is:

    the answer is "to the right of the lens".
    How the &**(#@ is this figured out? There are other questions similar to this one. I thought it was a prism type thing with rays going towards the base but it's not.
    I have no clue where to look for this. I am studying the Optical Math Tutorial.

    Can someone please help me? I am trying to get help from different sources and no one is biting.

    Thanks
    If you place an object at the primary focal point, the rays that diverge from this point will become parallel after refraction through the lens. The primary focal point of a diverging lens( minus) is on the right. On a converging lens(plus) it is on the left. Optical Formulas Tutorial has a ray tracing section, but you can see primary focal point on page 43-45.
    Also look at this link
    http://www.opticampus.com/cecourse.php?url=ray_tracing/
    Last edited by lensgrinder; 01-26-2016 at 07:00 AM. Reason: Added content

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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by lensgrinder View Post
    If you place an object at the primary focal point, the rays that diverge from this point will become parallel after refraction through the lens. The primary focal point of a diverging lens( minus) is on the right. On a converging lens(plus) it is on the left. Optical Formulas Tutorial has a ray tracing section, but you can see primary focal point on page 43-45.
    Also look at this link
    http://www.opticampus.com/cecourse.php?url=ray_tracing/
    What lensgrinder said, rays should come through a lens parallel, that's the big takeaway. Knowing that your knowledge of prism is accurate you could draw a representation of a plus or minus lens and then use your knowledge of prism to draw rays that are parallel on both sides of a lens and then deduce where the focal point would need to be to create those parallel rays. Look at the appendix from the link that Lensgrinder references, lots of good points to remember there, if you ray trace often this will become second nature. Draw out a few examples with graph paper and have at it, the only way to get a good intuition.
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    To better understand this question we must first assume some optical conventions…
    1) standard lens position has plus surface first, followed by minus surface on back of lens.
    2) all light rays move from left to right,
    3) one of the 5 ray tracing rules is … any light ray running parallel to axis is refracted by the lens through the lens’s secondary focal point (F’) on the axis.

    There is a primary focal point (F) and a secondary focal point (F’) in both plus and minus lenses. After passing through the plus or minus lens, where the emerging ray strikes the axis determines the position of the secondary focal point (F’).

    Passing through the lens:
    a) A light ray running parallel to the axis is converged by a plus lens to its secondary focal point (F’) on the axis to the right of the lens. Thus F’ is to the right of the plus lens.

    b) A light ray running parallel to the axis is diverged by a minus lens away from the axis. But if we extrapolate back this diverging ray, it will strike the axis in front of the minus lens at the minus lens’s secondary focal pointy (F’). Thus F’ is to the left of a minus lens.

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Nice...

    Quote Originally Posted by tmorse View Post
    To better understand this question we must first assume some optical conventions…
    1) standard lens position has plus surface first, followed by minus surface on back of lens.
    2) all light rays move from left to right,
    3) one of the 5 ray tracing rules is … any light ray running parallel to axis is refracted by the lens through the lens’s secondary focal point (F’) on the axis.

    There is a primary focal point (F) and a secondary focal point (F’) in both plus and minus lenses. After passing through the plus or minus lens, where the emerging ray strikes the axis determines the position of the secondary focal point (F’).

    Passing through the lens:
    a) A light ray running parallel to the axis is converged by a plus lens to its secondary focal point (F’) on the axis to the right of the lens. Thus F’ is to the right of the plus lens.

    b) A light ray running parallel to the axis is diverged by a minus lens away from the axis. But if we extrapolate back this diverging ray, it will strike the axis in front of the minus lens at the minus lens’s secondary focal pointy (F’). Thus F’ is to the left of a minus lens.
    Short and sweet.

    We need an "Eyeglasses & Optics for Dummies" (like me!). As a teacher you should look into it and I'm not kidding.

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    Quote Originally Posted by tmorse View Post
    To better understand this question we must first assume some optical conventions…
    1) standard lens position has plus surface first, followed by minus surface on back of lens.
    2) all light rays move from left to right,
    3) one of the 5 ray tracing rules is … any light ray running parallel to axis is refracted by the lens through the lens’s secondary focal point (F’) on the axis.

    There is a primary focal point (F) and a secondary focal point (F’) in both plus and minus lenses. After passing through the plus or minus lens, where the emerging ray strikes the axis determines the position of the secondary focal point (F’).

    Passing through the lens:
    a) A light ray running parallel to the axis is converged by a plus lens to its secondary focal point (F’) on the axis to the right of the lens. Thus F’ is to the right of the plus lens.

    b) A light ray running parallel to the axis is diverged by a minus lens away from the axis. But if we extrapolate back this diverging ray, it will strike the axis in front of the minus lens at the minus lens’s secondary focal pointy (F’). Thus F’ is to the left of a minus lens.
    On a closer reading of your question, you were actually looking for the placement of the ‘primary’ focal point (F) , and not the secondary focal point (F’) that I answered.

    Since the minus lens is transparent, light can pass from either direction (from the left or front to back surface convention, or from behind the lens and striking back surface to front surface).
    Adding another convention, the ‘primary’ focal point (F) is always found on the opposite side of the secondary focal point (F’), as if the parallel light ray was coming from the back, striking the minus surface first and then emerging through the front plus surface. The lens, still being minus, would again diverge this emerging ray, and by extrapolating back, the emerging ray would fall on the axis to the right of the minus lens at primary focus point (F) position.

    Sorry for the confusion.
    Last edited by tmorse; 01-31-2016 at 04:39 PM. Reason: highighting

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    OptiBoard Apprentice rmnrdi's Avatar
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    Click image for larger version. 

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    F1 is the primary focal point is F2 is the secondary focal point.

    Did you take the test yet?
    Last edited by rmnrdi; 01-03-2017 at 09:46 PM.
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