A plano-convex lens has thickness 4cm. When places on a horizontal table with the curved surface in contact with it, the apparent depth of the bottom-most point of the lens if found to be 3cm. If the lens is inverted such that the plane face of the lens is in contact with the table, the apparent depth of the center of the plane face of the lens is found to be `25//8` cm. Find the focal length of the lens.

Explanation

To solve the problem step by step, we will analyze the two cases of the plano-convex lens and use the information provided to find the focal length.

### Step 1: Understanding the Apparent Depth
In the first case, when the curved surface is in contact with the table, the apparent depth (D') of the bottom-most point of the lens is given as 3 cm. The thickness (D) of the lens is 4 cm. We can use the formula for apparent depth:

\[ D' = \frac{D}{\mu} \]
...