There may be various assessments that you have experienced at an eye exam and questioned what they are for. Having beams of light shined into your eyes may be one of them. This is one way eye doctors determine the refractive error of your eye, and it's called retinoscopy. Whether you're near or farsighted, or you have astigmatism, examining the way light reflects off your retina is a way your optometrist is able to see whether you need eyeglasses.
The most important thing your doctor is checking for during this exam is how accurately your eyes can focus. When light shines into your eye using a retinoscope, a reddish orange light reflects off your retina, through your pupil. Eye doctors call this the red reflex. The retinoscope measures your focal length, or in other words, to calculate the precise angle of refraction of light off your retina. And this is what lets us know how well your eye is able to focus. And if it's apparent that you aren't focusing properly, that's when we use a set of lenses. We hold up a few prescription lenses in front of your eye to determine which one will correct the refractive error. This is exactly how we calculate what prescription your glasses or contact lenses need to be.
The optometrist will perform your exam in a darkened room. You will usually be instructed to focus on an object behind the doctor. The exam doesn't include charts to be read, which means that a retinoscopy exam is also a very useful tool to determine the prescriptions of the speech-impaired, or young children.