For Carol, who is fitted with GP contact lenses on flat K and has a refraction of -5.00+2.00X 90, what is the effective power of her contact lenses?

To determine the effective power of the contact lenses for Carol, we need to take into account her refraction and the relationship between a rigid gas permeable (GP) lens and the corneal curvature represented by the flat K value.

Carol's refraction is -5.00 diopters with 2.00 diopters of astigmatism at an axis of 90 degrees. The GP lens is fit on the flat K value, which generally means that the lens will be aligned closely with the corneal curvature. For lens calculations, we commonly utilize the formula that accounts for the effect of the lens on the eye's overall refractive power.

In this scenario, the effective power of GP contact lenses can often be described as simply using the spherical equivalent of the refraction. The spherical equivalent is calculated by adding half of the cylinder value to the sphere. Here, half of 2.00 diopters (the cylinder) is 1.00 diopter, making the spherical equivalent:

-5.00 + 1.00 = -4.00 diopters.

However, since we are fitting the lens to the flat K value, adjustments due to the corneal topography must be taken into account as well. The effective power