There are some tests that you have seen during an eye exam and wondered what they are for. Having a bright light shined into your eyes may be an example. This is one way we determine 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 reflection of light off your retina is one test your eye doctor can use to see if you need eyeglasses.
The most important thing an eye doctor is looking for during this exam is how well your eyes can focus on the light. 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 determine your focal length, or in layman's terms, it will calculate the angle of refraction of light off your retina. And this is what lets us know how well your eye focuses. If it becomes clear that you can't focus well, we hold several lenses with varying prescriptions in front of the eye to determine which one will correct the refractive error. The lens power that works is the prescription you require to correct your sight with glasses or contact lenses.
All this happens in a darkened room. The patient will usually be instructed to focus on something ahead, just behind the doctor. This makes eyes easier to examine. Because a patient doesn't need to read eye charts during a retinoscopy exam, it's also a particularly useful way to determine an accurate prescription for kids who might struggle with speech, or others who might be speech-impaired.