Sometimes, particularly when performing an eye exam on small children the optometrist will shine a beam of light in the eye. But why? This test is a retinoscopy examination, and it's a basic way to determine the refractive error of your eye. Whether you're near or farsighted, or you have astigmatism, examining the way light reflects off your retina is one way your eye doctor can determine whether you need vision correction.
How well your eyes are able to focus under the circumstance we create during the exam is really what we're looking for. 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. We use the light to measure your focal length, or in simpler words, to determine the angle at which light refracts off your retina which lets us know how well your eye focuses. And if we notice that you aren't focusing correctly, we hold different prescription lenses in front of your eye to determine which one fixes the refractive error. And that is precisely how we find out the prescription your glasses or contact lenses need to be.
Your eye doctor will run your exam in a darkened room. To make your eyes easier to examine, you'll usually be told to keep your eyes fixed on an object behind the doctor. Unlike eye examinations you may have had, you won't be asked to read any charts. This means that a retinoscopy exam is also a very useful tool to determine the prescriptions of the speech-impaired, or young children.