Lensmaker's equation

For a given simple lens, the focal length $f$ can be calculated using the Lensmaker's Equation (also derived from Snell's law):

$$(n_2 - n_1)\left(\frac{1}{r_1} + \frac{1}{r_2}\right) = \frac{1}{f} .$$ (4.7)

The parameters $r_1$ and $r_2$ represent the radius of curvature of each of the two lens surfaces (front and back). This version assumes a thin lens approximation, which means that the lens thickness is small relative to $r_1$ and $r_2$. Also, it is typically assumed that $n_1 = 1$, which is approximately true for air.