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Thread: Internal lens reflection

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    Internal lens reflection

    Does it reflect off the last molecules of the lens material or the first molecules of the next medium? (be it either air or liquid)

    How does AR coating reduce such reflection?

    I always assumed that AR would reduce all reflection...but I was just wondering.... I know...should leave enough alone...

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    Internal lens reflection is from light bouncing around inside the lens. Once the angle of incidence (Outside light angle to surface it strikes) gets to 42 degrees you get total internal reflection. Total internal reflection is the light keeps bouncing around inside the lens and never exits. Don't aks me what happens to this energy and why it doesn't build up and explode as this has never been adequately explained to me.

    AR coat basicly is like a gravel surface on the lens that keeps light from bouncing back (like gravel keeps raindrops from bouncing up) but such small particulate that the light just passes through instead of being reflected (surface reflection, not internal reflection).

    Chip

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Giorio View Post
    Does it reflect off the last molecules of the lens material or the first molecules of the next medium? (be it either air or liquid)

    How does AR coating reduce such reflection?

    I always assumed that AR would reduce all reflection...but I was just wondering.... I know...should leave enough alone...
    Does it reflect off the last molecules of the lens material or the first molecules of the next medium? (be it either air or liquid)

    I would have to guess it bounces off the first molecule of the next medium, but chances are if your going down to this level of accurcay your not working in ophthalmic optics.

    How does AR coating reduce such reflection?

    Destructive Interference, google it.

    I always assumed that AR would reduce all reflection...

    Constructive Interference, google it.

    Quote Originally Posted by chip anderson
    Don't aks me what happens to this energy and why it doesn't build up and explode as this has never been adequately explained to me.
    Energy is absobed by the lens and dispersed into the air around it, if you used a laser the energy woulod build and melt the lens. For internal reflection we are working with the wave properties of light not the packets (photons) so technically we woudn't be doing anythign with the energy, now of course the light eventually leaves the lens somwhere often times in the lens the light leaves through the sides or in some cases will bounce of the sides of the lenses and leave back through the front of back surface causing the ring effect. It doesn't gather in the lens and explode though although it would be pretty cool if it did. Lasers kinda work in this way as they are reflected into the medium until they build enough energy to release packets (photons). It may not be the most elegant answer and may need more elaboration but that's what I got.

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    Master OptiBoarder rbaker's Avatar
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    Quote Originally Posted by chip anderson View Post
    Internal lens reflection is from light bouncing around inside the lens. Once the angle of incidence (Outside light angle to surface it strikes) gets to 42 degrees you get total internal reflection. Total internal reflection is the light keeps bouncing around inside the lens and never exits. Don't aks me what happens to this energy and why it doesn't build up and explode as this has never been adequately explained to me.

    Chip
    Total internal reflection is the principal on which fiber optics is base. A core of low index of refraction is "clad" with a higher index material.

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    Master OptiBoarder Darryl Meister's Avatar
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    Does it reflect off the last molecules of the lens material or the first molecules of the next medium? (be it either air or liquid)
    As Harry noted, light is technically "reflected" from the first molecules of the next medium. Essentially, photons of light are absorbed by the next medium and the re-emitted back in the opposite direction toward the first medium.

    I always assumed that AR would reduce all reflection...but I was just wondering.... I know...should leave enough alone...
    Antireflection coatings do not actually reduce all reflections; this is why you have that residual "reflex" color, particularly when viewing the coating at an angle.

    Don't aks me what happens to this energy and why it doesn't build up and explode as this has never been adequately explained to me
    Keep in mind that total internal reflection does not necessarily mean that light continues to bounce around in the material. Light is only totally internally reflected beyond certain angles of incidence at each surface. Eventually, the light will either exit the optical element (through transmittance or scattering) or become absorbed.
    Darryl J. Meister, ABOM

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