Correct Option
The correct option is One spherical surface.
Explanation
In ray optics, a lens is defined as a transparent optical medium bounded by two refracting surfaces. The defining characteristic of a lens, distinguishing it from a simple glass slab or prism, is the curvature of its bounding surfaces.
Detailed Analysis
- Definition of a Lens: According to standard optical definitions, a lens is a transparent material bound by two surfaces, of which at least one surface is spherical. The other surface may be either spherical or plane.
- Analysis of Options:
- One spherical surface is correct: This option aligns with the fundamental requirement that for a medium to act as a lens (converging or diverging light), it must possess at least one spherical surface. Examples include plano-convex or plano-concave lenses.
- Two plane surfaces only is incorrect: A transparent medium bounded by two plane surfaces parallel to each other is a rectangular glass slab. If the plane surfaces are inclined at an angle, it forms a prism. Neither is classified as a lens.
- One plane surface only is incorrect: A material cannot be bounded by "one plane surface only" to form a transmissive optical element like a lens; it requires a closed geometry with an entry and exit surface.
- Two cylindrical surfaces only is incorrect: While cylindrical lenses exist for specific applications (like correcting astigmatism), the general definition of a lens in fundamental physics relies on spherical surfaces. Furthermore, a lens does not require two cylindrical surfaces; it can be plano-cylindrical.
Key Takeaway: A lens is a transparent medium bounded by two surfaces, where at least one surface must be spherical. If both surfaces are plane, the object acts as a slab or prism, not a lens.