The amplifying medium can completely fill the space between the mirrors as the top of Figure \(\PageIndex{1}\), or there can be space between the amplifier and the mirrors. We state without proof that...The amplifying medium can completely fill the space between the mirrors as the top of Figure \(\PageIndex{1}\), or there can be space between the amplifier and the mirrors. We state without proof that in that case the distance \(L\) between the mirrors and the radii of curvature \(R_{1}\) and \(R_{2}\) of the mirrors has to satisfy \[0<\left(1-\frac{L}{R_{1}}\right)\left(1-\frac{L}{R_{2}}\right)<1, \nonumber \] or else the laser light will ultimately leave the cavity laterally, i.e.