On occasion, especially when doing an eye exam on small children the optometrist will focus a light in the eye. So what does this do? Such as test is used to help test the refractive error of your eye, and it's known as retinoscopy. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is one test your optometrist can employ to determine whether you need eyeglasses.
How well your eyes are able to focus during the exam is the main thing we look for. When we use the retinoscope to shine light into your eye, a reddish light reflects off your retina, through your pupil. Eye doctors call this the red reflex. The angle at which the 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. If it's apparent that you can't focus well, we hold up several lenses with varying prescriptions in front of the eye to determine which one will correct the refractive error. That lens power is the prescription you require to rectify your sight with glasses or contact lenses.
The optometrist will run your exam in a darkened room. To make your eyes easier to examine, you'll generally be instructed to keep your eyes fixed on an object behind the doctor. Unlike eye examinations you may have had, your doctor won't ask you to read any charts. This means that a retinoscopy exam is also a really good way to accurately determine the prescriptions of those who may struggle with speech, like young children and the elderly.