Problem 5.27

Determine the focal length of a concave mirror if: (a) with the distance between an object and its image being equal to \(l=15 \mathrm{~cm},\) the transverse magnification \(\beta=-2.0\); (b) in a certain position of the object the transverse magnification is \(\beta_{1}=-0.50\) and in another position displaced with respect to the former by a distance \(l=5.0 \mathrm{~cm}\) the transverse magnification \(\beta_{2}\) \(=-0.25\)