# Same focus for all circular lenses?

1. Oct 28, 2005

### InFiNitY1

In school physics i have learnt 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?

2. Oct 28, 2005

### James R

I think the formula f=r/2 only applies for idealised thin lenses. Hopefully, somebody else can provide more detail.

3. Oct 28, 2005

### Staff: Mentor

lens maker's formula

You are thinking of a concave (converging) mirror, not a lens.

Of course. The focal length of a lens depends on the refractive index as well as the curvature of the surfaces:
$$f = n (\frac{1}{R_1} - \frac{1}{R_2})$$

$R_1 , R_2$ are the radii of the surfaces. Look up the "lens maker's formula" to understand how this is used.

4. Oct 30, 2005

### James R

Thanks, Doc Al. Stupid of me not to realise that.