Visual Optics Practice Test 2025 - Free Visual Optics Practice Questions and Study Guide

To determine the power needed in spectacles for a 2.5 D myope at a 40 cm working distance, it's essential to understand how to calculate the required spectacle lens power based on the individual's refractive error and the desired working distance.

A myope (nearsighted individual) has a negative refractive error that requires the use of diverging lenses (negative power) to correct their vision. The power of the lens needed can be calculated using the lens formula and the distance at which the patient wants to focus.

In this case, a 2.5 D myope would naturally focus at a far point. The far point for this patient would be at approximately 40 cm (since 1/2.5 D = 40 cm). To see clearly at this distance, they need a lens that compensates for this myopia and allows for clear vision at 40 cm.

For optimal vision correction, the formula used is:

Power (D) = -(distance in meters) + (deficiency in diopters).

Here, the distance is 0.4 meters (40 cm), and the myopia is -2.5 D:

Power needed = -2.5 D - (1 / 0.