The figure shows a parallel beam of width $6.0\ \text{cm}$ that falls on a convex lens L1. A second thin lens L2, placed $20\ \text{cm}$ beyond L1, sends the light out as a parallel beam of width $2.0\ \text{cm}$. The second lens stands in front of the point where L1 would bring the light to a focus. What are the focal lengths of L1 and L2, in that order?

Fig. 1: Two thin lenses on a horizontal axis. Five parallel rays make a beam of width 6.0 cm that comes in from the left and falls on a convex lens L1. The rays converge after it and fall on a second lens L2, drawn as a concave lens, 20 cm beyond L1. After L2 the five rays are parallel again and make a beam of width 2.0 cm. Dimension arrows give the two widths and the 20 cm gap. The figure is not to scale.