Same focus for all circular lenses?

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    Circular Focus Lenses
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Discussion Overview

The discussion revolves around the focal properties of circular convex lenses, particularly in relation to their material composition and refractive indices. Participants explore the implications of using different materials, such as glass and perspex, on the focal length of identical lenses.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant notes that the formula f=r/2 for the focus of a circular convex lens may not hold for lenses made from different materials due to variations in refractive index.
  • Another participant suggests that the formula f=r/2 is applicable only to idealized thin lenses and invites further clarification from others.
  • A third participant points out that the focal length of a lens is influenced by both the refractive index and the curvature of the lens surfaces, referencing the lens maker's formula f = n (1/R1 - 1/R2).
  • A later reply acknowledges a misunderstanding regarding the type of optical element being discussed, clarifying that the initial formula was mistakenly attributed to a concave mirror rather than a lens.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the formula f=r/2 and its limitations, indicating that multiple competing perspectives remain regarding the focal properties of lenses made from different materials.

Contextual Notes

The discussion highlights the dependence of focal length on both the refractive index and the curvature of the lens surfaces, suggesting that assumptions about ideal conditions may not apply universally.

InFiNitY1
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In school physics i have learned that the focus for a circular convex(converging) lens is f=r/2 where r is the radius and f is focus. But wouldn't a pair of identical lens made of a different material eg glass and perspecs which have different refractive indexes have different point of focus?
 
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I think the formula f=r/2 only applies for idealised thin lenses. Hopefully, somebody else can provide more detail.
 
lens maker's formula

InFiNitY1 said:
In school physics i have learned that the focus for a circular convex(converging) lens is f=r/2 where r is the radius and f is focus.
You are thinking of a concave (converging) mirror, not a lens.

But wouldn't a pair of identical lens made of a different material eg glass and perspecs which have different refractive indexes have different point of focus?
Of course. The focal length of a lens depends on the refractive index as well as the curvature of the surfaces:
[tex]f = n (\frac{1}{R_1} - \frac{1}{R_2})[/tex]

[itex]R_1 , R_2[/itex] are the radii of the surfaces. Look up the "lens maker's formula" to understand how this is used.
 
Thanks, Doc Al. Stupid of me not to realize that.
 

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