Q116 / 125 · BITSAT 2018 · Physics
A planoconvex lens has thickness of When placed on a horizontal table, with the curved surface in contact with it, the apparent depth of the bottom most point of the lens is found to be If the lens is inverted such that the plane face is in contact with the table, the apparent depth of the centre of the plane face is found to be The focal length of the lens is (assume thickness of lens to be negligible)
- A
- B
- Correct answerC
- D
Solution
In first case refraction of the rays, takes place from a plane surface, so we can use
where, and apparent and real depth, respectively.
Now, in second case, as from Fig. (b), refraction takes place from a spherical surface, so
or
or
Now, using Lens' Makers formula, to calculate focal length
Hence, the correct answer is option: 'C'.