On occasion, particularly when performing an eye exam on a small child the eye doctor will shine a beam of light in the eye. But why? This test is called a retinoscopy examination, which is a preliminary way to assess the refractive error of your eye. By merely examining the way light reflects off your retina, the optometrist can assess whether you are nearsighted, farsighted or have astigmatism. This is how they may also get a pretty good reading on the prescription required to correct your vision.
The main thing your doctor is looking for during this exam is how well your eyes can focus on the light. When light shines into your eye using a retinoscope, a reddish light reflects off your retina, through your pupil. Eye doctors call this the red reflex. The angle at which the light reflects off your retina, also called your focal length, is the thing that tells us how well your eye can focus. If it becomes clear that you are not focusing well, we hold up several lenses with varying prescriptions in front of your eye to determine which one will correct the refractive error. And that is exactly how we find out what prescription your glasses or contact lenses need to be.
The eye doctor will perform your exam in a darkened room. To make your eyes easier to examine, you'll usually be instructed to focus on an object behind the doctor. Not having to read any eye charts means that a retinoscopy exam is also a very good way to accurately determine the prescriptions of the speech-impaired, or young children.