Final answer:
Gas-permeable scleral lenses are primarily indicated for keratoconus and irregular corneas. They improve vision by creating a regular optical surface over the irregular cornea. Cataracts diffuse light, while laser eye surgery requires parallel rays for precision.
Step-by-step explanation:
The two primary indications for gas-permeable scleral lenses are for the treatment of keratoconus and irregular corneas. These lenses are beneficial because they vault over the cornea and rest on the white part of the eye (the sclera), creating a tear-filled chamber over the irregular cornea. This design allows for a more regular optical surface, which can significantly improve vision in patients with corneal irregularities, such as those caused by keratoconus, where the cornea has a cone-like bulge. They also provide a solution for managing complex corneal surface irregularities which cannot be adequately corrected with standard contact lenses or eyeglasses.
As for the concepts covered in the question, a cataract is indeed cloudiness in the lens of the eye, and this cloudiness causes light to be dispersed or diffused, leading to blurred vision. When performing laser surgery on an eye, such as spot-welding the retina to the back of the eye, the rays entering the eye must be parallel in order to focus the laser accurately on the retina without damaging surrounding tissues.