# Formula Biconcave Lens Focal length    Refractive index of the medium    Right radius of curvature

## Focal length

Unit
Focal length is the distance between the lens (or the principal plane for a thick lens) and the focal point. For a biconcave lens, the focal length is negative. For a biconvex lens it is positive.

## Refractive index of the medium

Unit
Refractive index of the medium outside the lens. If the lens is in the air, then $$n_{\text M} \approx 1$$.

## Refractive index of the lens

Unit
Refractive index of the material from which the lens is made.

Right radius of curvature $$R_1$$ of the biconcave lens (see illustration).
Left radius of curvature $$R_2$$ of the biconcave lens (see illustration).