There may be some assessments that you have seen at an eye exam and questioned how they work. Having beams of light shined into your eye could be an example. Such as test is used to help test the refractive error of your eye, and it's known as retinoscopy. By looking at the reflection of light off your retina, the eye care professional can decide whether you are nearsighted, farsighted or have astigmatism. This is how they can also get a pretty good reading on the prescription you would need to correct your vision.
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 orange light reflects off your retina, through your pupil. We call this the red reflex. The retinoscope measures your focal length, or in simpler words, to determine the precise angle of refraction of light off your retina. And this is what lets us know how well your eye focuses. If it becomes obvious that you can't focus well, that's when we use a set of lenses. We hold a variety of lenses with varying prescriptions in front of the eye to determine which one fixes the refractive error. This is exactly how we find out the prescription your glasses or contact lenses need to be.
These exams are generally conducted in a dark room. To make your eyes easier to examine, you'll generally be asked to focus on an object behind the doctor. The exam doesn't include eye charts, which means that a retinoscopy exam is also a very useful tool to determine the prescriptions of the speech-impaired, or young children.