Question

an object of 2cm in size is placed 30cm in front of the concave mirror of focal length15cm. at what distance from the mirror should screen be placed in order to obtain a sharp image

Answer 1

Size of the object, h = +2 cm
Object distance, u = -30 cm

Focal length, f = -15 cm (Concave mirror)
Image distance, v = ?
Size of the image, h' = ?

By using mirror formula :

$$\begin{gathered} \frac{1}{v}+\frac{1}{u}=\frac{1}{f} \\ \Rightarrow \frac{1}{v}+\frac{1}{-30}=\frac{1}{-15}\mathbf{\Rightarrow }\mathit{v}\mathbf{}\mathbf{=}\mathbf{}\mathbf{-}\mathbf{30}\mathbf{}\mathit{c}\mathit{m}Now,Magnificantion,M\hspace{0.17em}=-\frac{v}{u}=\frac{-(-30)}{-30}=-1\Rightarrow M\hspace{0.17em}=\frac{h\text{'}}{h}=-1\mathbf{\Rightarrow }\mathit{h}\mathbf{\text{'}}\mathbf{}\mathbf{=}\mathbf{}\mathbf{-}\mathbf{2}\mathbf{}\mathit{c}\mathit{m} \end{gathered}$$