1. v_{e} = -25 cm

\(\frac{1}{u_e}=\frac{1}{v_e}-\frac{1}{f_e}\) = \(\frac{1}{-25}-\frac{1}{6.25}\)

∴ u_{0} = 5 cm

Length of the tube, L= |v_{0}| + |u_{e}|

∴ v_{0} = 15 – 5 = 10

\(\frac{1}{u_0}=\frac{1}{v_0}-\frac{1}{f_0}\) = \(\frac{1}{10}-\frac{1}{2}\)

u_{e} = -2.5 cm.

2. ∴ v_{0} = 15 – f_{e }= 15 – 6.25 = 8.75

\(\frac{1}{u_0}=\frac{1}{v_0}-\frac{1}{f_0}\)= \(\frac{1}{8.75}\) - \(\frac{1}{2}\)

u_{0} = -2.59 cm