Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Optics - Glass lenses in other mediums

  1. Apr 17, 2005 #1
    Glass lenses behave in the way it is expected to behave when the medium they are in is a vacuum or air. But if the medium is also glass it obviously doesn’t work as a lens. If the density of the medium is higher than that of glass the lens works in the opposite way.

    The power of a lens depends on the medium it is in. So if we consider a lens that’s sunk in water it’s not as powerful as it was when the medium was air.

    The power of a lens= 1/focal length

    When the power decrease the focal lens should increase.
    But as “2F=C” the c should also increase.
    Yet c is defined as “The radius of the glass sphere that was used to make the lens”
    If the definition is correct C cannot be changed.

    Either the definition is wrong or the principals we use to specify the path of rays of light are only valid when the medium is air or a vacuum.
  2. jcsd
  3. Apr 18, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    Where does 2F = C come from? There is no way to come up with the focal lenth of a lens without considering the relative index of refraction of the lens material and the medium. The usual equations relating focal length to lens curvature include the index of refraction.
  4. Apr 18, 2005 #3
    If you read your own post close enough, your first paragraph is inconsistent with "2F = C". You need to differentiate between the actual focal points, centers and principal planes (in case of thin lenses, you don't have to worry about the principal planes) and the focal points, centers and principal planes which are geometric in origin (that is the physical properties due to the lens' construction).

    When a lens or a mirror is immersed in a medium which has a refractive index not equal to unity (relative) the observed behavior of the lens is actually that of the optical system composed by the lens and the medium. You cannot say that the rays incident parallel to the principal axis will converge to the same focal point as given to you by the manufacturer. As OlderDan has nicely mentioned, these properties will in general change with the nature of the interaction medium (i.e. the environment).

    So you have to be careful about geometric parameters and actual (experimentally observed) parameters. This is like saying (in simple terms) that the focal length of a convex lens (or mirror) is 45cm in air but it could be 20cm in water (forget about the actual values as this is just an example). To avoid confusion, we define the focal points as points where rays in the external medium incident parallel to the optical axis converge after refraction from the lens. Clearly this definition is better than saying that the lens which is spherical has its center of curvature at such and such place and so its focal length should be half the distance of the pole to this center. Nobody is going to prevent you from saying that but then, this location will be inconsistent with the properties of the focal point that are used to define it in practise.

    Hope that helps...


    EDIT: You might like to view other geometric optics posts on PF for some questions, queries and ideas. Additionally from my personal experience, optics can be a pain if its not done properly. It really is damn easy but you need to get over all the hype about sign conventions first. For this, I recommend Resnick, Halliday, Krane volume 2.
  5. Apr 18, 2005 #4
    Thanks for all your help,got it figured out.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook