Sometimes, particularly when performing an eye exam on small children the optometrist will direct a beam of light in the eye. But what does this do? Firstly, this test is known as 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 one way your eye doctor can determine whether you need eyeglasses.
The main thing your doctor is looking for during this exam is how well your eyes can focus on the light. 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. We use the light to determine your focal length, or in simpler words, to measure the precise angle at which light refracts off your retina which lets us know how well your eye focuses. If it's apparent that you can't focus properly, we hold up a variety of prescription lenses in front of your eye to determine which one fixes the refractive error. This is exactly how we find out what prescription your glasses or contact lenses need to be.
The retinoscopy exam is performed in a dark or dimmed room. The patient will usually be told to focus on something behind the doctor. This makes eyes easier to examine. 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 useful tool to determine the prescriptions of the speech-impaired, or young children.