Explanation:
Power \(=\dfrac{1}{\text { Focal length }}\)
Focal length of combination of convex
and concave lenses is given by
\(\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{1}{F} = \frac{1}{{{f_1}}} + \frac{1}{{{f_2}}}\)
where, \(f_{1}\) and \(f_{2}\) be the focal lengths of
convex and concave lenses, respectively.
Now, \(\,\,\,\,\,\,\,\,\,\,\,\frac{1}{F} = \frac{1}{{0.4}} + \frac{1}{{( - 0.25)}}\)
\( \Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{1}{F} = - 1.5\)
Power, \(\,\,\,\,\,\,\,\,\,\,P = - 1.5D\)
