If there are two identical lenses L1 and L2 but made with different materials such that refractive index of one is greater than the other (Say for L1 = 1.33 and L2 = 2.13). Will it affect in any way? Is there a change in focal length or where the image is formed? Please explain.

  • 5
    $\begingroup$ Yes, the focal length is dependent on the optical density of the material. See- Lens Maker's Formula $\endgroup$
    – Sam
    Jan 18, 2020 at 13:01

1 Answer 1


A lens works because it has a different refractive index from the medium it is in. The bigger that difference, the stronger the lens is for a given shape.

That also means the focal length of a glass lens in air is less than its focal length in water. The smaller difference in refractive index makes it weaker.

  • $\begingroup$ So, the focal length doesn't have to be half of the Radius of curvature (I have seen the relation R= 2f )? $\endgroup$ Jan 18, 2020 at 16:09
  • 2
    $\begingroup$ The relationship $R=2f$ is for a concave mirror, not for a lens. $\endgroup$ Jan 18, 2020 at 16:56

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