Sometimes, particularly when doing an eye exam on small children the optometrist will focus a light in the eye. But what does this do? This is one way we determine the refractive error of your eye, and it's known as a retinoscopy exam. By examining the way light reflects off your retina, the eye care professional can determine if you are nearsighted, farsighted or have astigmatism. This is how they can also get a pretty good reading on the prescription you would need to correct your vision.
In short, what we are doing during a retinoscopy exam is checking to see how your eye can focus. We shine light into your eye because we are looking for what we call the red reflex. The retinoscope aims light into your eye, and a red or orange light reflects through your pupil and off your retina. The angle at which the retinoscope's light reflects 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. If it becomes clear that you can't focus properly, that's when we use a set of lenses. We hold up a few lenses with varying prescriptions in front of your eye to see which one rectifies your vision.
Your eye doctor will run your exam in a dark or dimmed room. To make your eyes easier to examine, you'll generally be told to look at an object behind the doctor. Because a patient isn't instructed to read eye charts during a retinoscopy exam, it means that it's also a really great way to determine the prescriptions of kids who might struggle with speech, or others who might be speech-impaired.