The principal meridians of a lens are always positioned how?

The principal meridians of a lens refer to the directions in which the lens has the least and greatest power. In any cylindrical lens, which can be used to correct astigmatism, these principal meridians are always positioned 90 degrees apart from each other.

This configuration allows the lens to focus light more effectively by compensating for the different refractive powers in the perpendicular axes of the lens. The arrangement ensures that one meridian has maximum power, while the meridian at 90 degrees to it has minimum power, creating the necessary correction for astigmatism.

Consequently, this 90-degree separation between the principal meridians is fundamental to the lens's design and functioning, making it a critical concept in optics and ophthalmology. The other referenced options do not accurately reflect the relationship between the principal meridians of a lens.