There are some tests that you have seen at an eye exam and questioned what they are for. Having a bright light shined into your eyes may be an example. Such as test is used to help test the refractive error of your eye, and it's called a retinoscopy exam. Whether you're near or farsighted, or you have astigmatism, examining the way light reflects off your retina is a test your eye doctor can employ to see if you need eyeglasses.
The most important thing your doctor is looking for during this exam is how accurately 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. The degree at which the retinoscope's light refracts off your retina, also called your focal length, is precisely what lets us know how well your eye can focus. If it becomes clear that you can't focus properly, we hold different prescription lenses in front of the eye to determine which one fixes the refractive error.
All this happens in a dark or dimmed room. The patient will usually be instructed to focus on something ahead, just behind the doctor. Not having to read any eye charts means that a retinoscopy exam is also a really useful tool to determine the prescriptions of those who may struggle with speech, like young children and the elderly.