Lens maker’s formula for a double concave lens. Equation 24.32 gives the focal length of a double convex lens (i.e., a lens for which both surfaces are convex) in terms of the radii of curvature of the two surfaces. For a double concave lens (Fig. P24.85), the lens maker’s formula has a slightly different form: 

This relation always gives a negative focal length f as expected for a double concave lens (because it is a diverging lens). Consider such a lens composed of quartz with R₁ = R₂ = 50 cm and n = 1.56 for blue light. What is its focal length for blue light?