Question

An object 20 cm from a spherical mirror gives rise to a virtual image 15 cm behind the mirror. The type of the mirror and its focal length is
(a) concave, 8.5 cm
(b) convex, 30 cm
(c) concave, 60 cm
(d) convex, 60 cm.

Answer 1

$$\begin{gathered} DearStudent, \\ Given, \\ u=-20cm,v=+15cm(Imageisformedbehindthemirror.) \\ \frac{1}{f}=\frac{1}{u}+\frac{1}{v} \\ \Rightarrow \\ \frac{1}{f}=-\frac{1}{20}+\frac{1}{15}=\frac{-3+4}{60}=+\frac{1}{60} \\ \Rightarrow \\ f=60cm \\ Asthefocallengthispositive,themirrorusedisconvex. \\ \mathit{A}\mathit{n}\mathit{s}\mathit{w}\mathit{e}\mathit{r}\mathbf{}\mathbf{:}\mathbf{}\left(d\right)\mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{x}\mathbf{,}\mathbf{60}\mathit{c}\mathit{m} \\ Regards. \end{gathered}$$