Statement I : Crookeӳ glass for spectacles contains rare earth metal oxides of praseodymium and neodymium. Statement II Crookeӳ glass is characterized by its ability to absorb ultraviolet light and to transmit visible light completely.

The correct answer is Option 4 because Statement I is scientifically inaccurate, while Statement II correctly describes the functional properties of Crooke’s glass.

Analysis:

• Statement I is false: Crooke’s glass primarily contains cerium oxide (CeO2) along with other metallic oxides. While praseodymium and neodymium are used in "Didymium glass" to filter specific wavelengths (like sodium flare), they are not the defining components of standard Crooke’s glass used for UV protection.
• Statement II is true: The defining characteristic of Crooke’s glass is its specialized chemical composition that allows it to effectively absorb ultraviolet (UV) rays. Simultaneously, it maintains a high refractive index and transparency, allowing visible light to pass through almost completely, thereby protecting the eyes without significantly impairing vision.

Since the first statement misidentifies the primary rare earth element and the second statement accurately describes the glass's utility, Option 4 is the only valid choice.