During your eye exam, your eye doctor might shine a beam of light into your eye, and hold various lenses in front of it. But what does this do? Firstly, 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 reflection of light off your retina is a test your eye doctor can use to determine if you need eyeglasses.
How well your eyes focus under the circumstance we create during the retinoscopy exam is really what we're looking for. When we use the retinoscope to shine light into your eye, a reddish light reflects off your retina through your pupil. We call this the red reflex. The degree at which the retinoscope's light refracts off your retina, which is what eye care professionals call your focal length, is precisely what tells us how well your eye can focus. And if we see that you are not focusing correctly, that's when we use a set of lenses. We hold up a number of lenses with varying prescriptions in front of the eye to see which one rectifies the error. That lens power is the prescription you will need to correct your sight with glasses or contact lenses.
All this happens in a darkened room. You will usually be instructed to look at an object behind the doctor. This makes eyes easier to examine. 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.