Converging Lens

A coupler or converging lens is a type of optical lens that changes a parallel beam of rays into a converging one.

Basic information
We distinguish couplers (in general and lenses) into thin and thick.


 * A thin coupler is one in which its thickness is negligible compared to its focal length f.
 * A thick coupler is the opposite of a thin one, i.e. we have to take its thickness into account. (cannot be neglected)

If we consider the sign convention, the radii of curvature of the lens are positive for convex surfaces and negative for hollow surfaces.

If n 2 > n 1 (e.g. for a lens in air) f > 0 applies to couplings, for dispersions the opposite values ​​are f < 0.

Optical power
The reciprocal of the focal length f is called the optical power M.

$$ M = \frac{1}{f}$$

The unit of optical power is Diopter [D]; 1 D is the optical power of a lens with a focal length of 1 m. We usually use it to describe the display of thin lenses.

Imaging equation for a thin lens
$$ \frac{1}{f} = \frac{1}{a}+\frac{1}{b} $$

$$ f = \frac{ab}{a+ b} $$

Related Articles

 * Correction of refractive errors
 * Optical Power