The least distance of distinct vision of an average person (i.e., $D$ ) is 25 cm , in order to view an object with magnification 10,

Here, $v=D=25 \mathrm{~cm}$ and $u=f$

But the magnification $m=v / u=D / f$

$$\begin{aligned} & m=\frac{D}{f} \\ \Rightarrow\quad & f=\frac{D}{m}=\frac{25}{10}=2.5=0.025 \mathrm{~m} \\ & P=\frac{1}{0.025}=40 \mathrm{D} \end{aligned}$$

This is the required power of lens.

No, the reversibility of the lens maker's equation.

$$\begin{aligned} &\text { Let the apparent depth be } O_1 \text { for the object seen from } m_2 \text {, then }\\ &O_1=\frac{\propto_2}{\propto_1} \frac{h}{3} \end{aligned}$$

Since, apparent depth $=$ real depth $/$ refractive index $\propto$.

Since, the image formed by Medium $1, \mathrm{O}_2$ act as an object for Medium 2.

If seen from $\propto_3$, the apparent depth is $\mathrm{O}_2$.

Similarly, the image formed by Medium $2, \mathrm{O}_2$ act as an object for Medium 3

$$ \begin{aligned} O_2 & =\frac{\propto_3}{\propto_2}\left(\frac{h}{3}+O_1\right) \\ & =\frac{\propto_3}{\propto_2}\left(\frac{h}{3}+\frac{\propto_2 h}{\propto_1 3}\right)=\frac{h}{3}\left(\frac{\propto_3}{\propto_2}+\frac{\propto_2}{\propto_1}\right) \end{aligned}$$

Seen from outside, the apparent height is

$$\begin{aligned} O_3 & =\frac{1}{\propto_3}\left(\frac{h}{3}+O_2\right)=\frac{1}{\propto_3}\left[\frac{h}{3}+\frac{h}{3}\left(\frac{\propto_3}{\propto_2}+\frac{\alpha_3}{\propto_1}\right)\right] \\ & =\frac{h}{3}\left(\frac{1}{\propto_1}+\frac{1}{\propto_2}+\frac{1}{\propto_3}\right) \end{aligned}$$

This is the required expression of apparent depth.

The relationship between refractive index, prism angle $A$ and angle of minimum deviation is given by

$$\propto=\frac{\sin \left[\frac{\left(A+D_m\right)}{2}\right]}{\sin \left(\frac{A}{2}\right)}$$

Here,

$\therefore$ Given, $$D_m=A$$

Substituting the value, we have

$$\begin{aligned} & \therefore \quad \alpha=\frac{\sin A}{\sin \frac{A}{2}} \\ & \text { On solving, we have } \quad =\frac{2 \sin \frac{A}{2} \cos \frac{A}{2}}{\sin \frac{A}{2}}=2 \cos \frac{A}{2} \end{aligned}$$

$$\propto=\frac{\sin A}{\sin \frac{A}{2}}=\frac{2 \sin \frac{A}{2} \cos \frac{A}{2}}{\sin \frac{A}{2}}=2 \cos \frac{A}{2}$$

For the given value of refractive index, we have

$$\begin{array}{llr} \therefore & \cos \frac{A}{2}=\frac{\sqrt{3}}{2} \\ \text { or } & \frac{A}{2} =30 \Upsilon \\ \therefore & A =60 \Upsilon \end{array}$$

This is the required value of prism angle.

Since, the object distance is $u$. Let us consider the two ends of the object be at distance $u_1=u-L / 2$ and $u_2=u+L / 2$, respectively so that $\left|u_1-u_2\right|=L$. Let the image of the two ends be formed at $v_1$ and $v_2$, respectively so that the image length would be

$L^{\prime}=\left|v_1-v_2\right|\quad\text{.... (i)}$

Applying mirror formula, we have

$$\frac{1}{u}+\frac{1}{v}=\frac{1}{f} \text { or } v=\frac{f u}{u-f}$$

On solving, the positions of two images are given by

$$v_1=\frac{f(u-L / 2)}{u-f-L / 2}, v_2=\frac{f(u+L / 2)}{u-f+L / 2}$$

For length, substituting the value in (i), we have

$$L^{\prime}=\left|v_1-v_2\right|=\frac{f^2 L}{(u-f)^2 \times L^2 / 4}$$

Since, the object is short and kept away from focus, we have

$$\begin{gathered} L^2 / 4<<(u-f)^2 \\ \text{Hence, finally}\quad L^{\prime}=\frac{f^2}{(u-f)^2} L \end{gathered}$$

This is the required expression of length of image.