What tool shines a beam of light onto the retina to allow the OD to examine its movement?

The retinoscope is the correct tool used to shine a beam of light onto the retina, enabling the optometrist (OD) to assess its movement. The fundamental purpose of a retinoscope is to determine the refractive error of a patient's eye. When the beam of light is directed at the retina, the reflection and movement of the light indicate how the eye is focusing light. By observing the direction of the light's movement as it interacts with the retina, the optometrist can evaluate whether the eye is myopic (nearsighted), hyperopic (farsighted), or emmetropic (normal vision).

This tool is essential during refraction tests, and its ability to provide real-time feedback about the retina's response to light is what makes it incredibly valuable in clinical practice. The other tools mentioned do have their uses in the field of optometry but serve different purposes. For instance, the fundoscopic tool examines the internal structures of the eye but is not specifically used to evaluate retinal movement.