What is the back vertex power of a lens measured when the concave side of a gas permeable lens is placed against the lens stop of the lensometer?

The back vertex power of a lens is defined as the power measured when the concave side of the lens is placed against the lens stop of the lensometer. This measurement is important because it provides the effective optical power of the lens as it relates to the eye, especially when determining how the lens will interact with the light entering the eye.

When the lens is oriented in this manner, the measurement taken reflects the power at the rear vertex of the lens, which is vital for ensuring proper fitting and vision correction. The concave surface facing the lens stop guarantees that the light rays will pass through the lens appropriately, allowing for an accurate power reading.

Understanding this concept is essential for fitting gas permeable contact lenses, as the back vertex power directly affects how effectively the lens corrects for refractive errors. It is crucial for practitioners to be able to measure and interpret this value accurately to ensure optimal vision correction and comfort for the wearer.

Other options, such as front vertex power or base curve power, refer to different measurements and contexts that do not align with the specific arrangement described in the question. Therefore, recognizing that the back vertex power is the correct answer in this scenario helps in grasping fundamental concepts in lensometry and contact lens fitting.