American Board of Opticianry (ABO) Practice Test 2026 – The All-in-One Guide to Master the American Board of Opticianry Exam!

The cornea is a transparent, dome-shaped structure that forms the front surface of the eye. It is responsible for refracting or bending light that enters the eye, providing about 70-75% of its focusing power. The other options, such as the iris, sclera, and retina, do play important roles in the eye's function, but they do not supply the majority of its focusing power. The iris controls the size of the pupil and the amount of light entering the eye, the sclera is the white outer layer of the eyeball that provides structure and protection, and the retina is the light-sensitive tissue at the back of the eye that contains photoreceptor cells for vision. However, the cornea is the main structure responsible for focusing light onto the retina, making it the correct answer to the question.