During your eye exam, you may have had a doctor shine a beam of light into your eye, and hold various lenses in front of it. But what does this do? This test is known as a retinoscopy examination, and it's a basic way to assess the refractive error of your eye. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is one way your eye doctor is able to determine whether you need eyeglasses.
Basically, what we are doing during the retinoscopy exam is checking to see how your eye focuses. We shine light into your eye because we are looking for what's known as your red reflex. The retinoscope aims a beam of light into your eye, and a red or orange light reflects through your pupil and off your retina. The degree at which the light reflects off your retina, also called your focal length, is exactly what lets us know how well your eye can focus. And if we notice 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 determine which one fixes the refractive error. This is exactly how we calculate the prescription your glasses or contact lenses need to be.
The eye doctor will run your exam in a dark or dimmed room. You will usually be instructed to focus on something behind the doctor. This makes eyes easier to examine. Not having to read any eye charts means that a retinoscopy exam is also a really good way to accurately determine the prescriptions of the speech-impaired, or young children.