Certified Paraoptometric (CPO) Practice Exam 2025

Which type of lens is thin in the center and thick on the edges, causing objects to appear smaller?

1. Plus lenses

2. Minus lenses

3. Cylindrical lenses

4. Aspherical lenses

The correct answer is: Minus lenses

The correct answer is that minus lenses are thin in the center and thick at the edges. These lenses diverge light rays, which results in a virtual image that appears smaller than the actual object. This property makes them especially effective for correcting nearsightedness (myopia), where distant objects appear blurry. By spreading out the light entering the eye, minus lenses help individuals see these objects more clearly. Other types of lenses, such as plus, cylindrical, or aspherical lenses, have different shapes and functions. Plus lenses, for instance, are thicker in the center and can magnify objects, which is the opposite effect of minus lenses. Cylindrical lenses are designed to correct astigmatism by having different curvature in different meridians, and aspherical lenses provide a more complex curvature to reduce optical distortions. Each type of lens serves a specific corrective purpose, making it crucial to understand their characteristics and applications.