Sometimes, especially when doing an eye exam on small children the optometrist will shine a light in the eyes. But why? This is one way eye doctors determine the refractive error of your eye, and it's called retinoscopy. By looking at the way light reflects off your retina, the eye doctor can determine 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 most important thing your doctor is looking for during this exam is how well your eyes can focus. When we use the retinoscope to shine light into your eye, a reddish light reflects off your retina, through your pupil. This is called the red reflex. The angle at which the retinoscope's light refracts off your retina, which is what eye care professionals call your focal length, is the thing that lets us know how well your eye can focus. And if we notice that you are not focusing well, we hold up a variety of lenses with varying prescriptions in front of your eye to determine which one will correct the refractive error. This is precisely how we find out the prescription your glasses or contact lenses need to be.
The retinoscopy exam is usually performed in a dark or dimmed room. To make your eyes easier to examine, you'll generally be instructed to keep your eyes fixed on something 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.