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Optics: Spherical Interface -- Real and Virtual Images

  1. Aug 31, 2014 #1
    1. The problem statement, all variables and given/known data

    If you have a spherical interface between two different "media" (like air and water), and an object is placed in the one with the lower index of refraction, with the interface being convex toward the object, how can you tell if the image will be real or virtual?

    Here's a picture of the two different possibilities. The white area has a lower index of refraction than the blue area, so the rays will bend toward the normal when they strike it. But how far they bend determines if the image will be real or virtual. I know about Snell's Law, but I think it would be difficult to apply here since the interface is not flat. Also, I am pretty sure whether the image is real or virtual depends on the distance the object is from the interface, but I don't know how to find that distance.


    2. Relevant equations

    I'm sure there is a simple one somewhere but I can't find it in my notes or book :(

  2. jcsd
  3. Aug 31, 2014 #2


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  4. Aug 31, 2014 #3
    Thanks for the link! So it all just comes from Snell's Law? That page doesn't have any example of a case where the interface is convex but the image is virtual, however this must happen at least sometimes, since the thin-lens formula is derived from the first interface giving a virtual image.

    I just found something in my notes talking about the focal length, and if the object's distance from the interface is less than the focal length, then the image will be virtual. I'll look up how to find what this focal length is. Thanks!
  5. Aug 31, 2014 #4


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    The focus is the image point when the object is at infinity. If the object is at the other focus, the refracted rays are parallel with the principal axis, so the image distance is infinity. You find the relation between object distance and image distance at that URL. I am sure, you also find these topics in your lecture notes.
    Or browse "refraction at spherical surfaces". You find lot of places.

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