Final answer:
Parallel rays are needed in laser eye surgery to simplify the focusing of the laser on the retina, ensuring precise and small burns for accurate repair and efficient coagulation, especially when using an argon laser which is strongly absorbed by blood-containing tissues.
Step-by-step explanation:
When laser light is shone into a relaxed normal-vision eye to repair a tear by spot-welding the retina to the back of the eye, the rays entering the eye must be parallel to simplify the focusing of the laser on the retina.
Parallel rays help to ensure that the laser point is small and precise, which is necessary for the laser to accurately create scars and to efficiently coagulate the torn area.
The accuracy is crucial because the laser must be focused to a small spot on the retina to form scar tissue that holds the retina in place, saving the patient's vision. A green argon laser is typically used in this procedure because its light is absorbed well by tissues containing blood, which aids in the coagulation process. Laser surgery is highly preferred over other light sources due to its precision, which is attributed to the minimal refractive dispersion of its single wavelength.