There are various assessments that you have seen during an eye exam and questioned how they work. Having beams of light shined into your eyes may be an example. This is one way eye doctors test the refractive error of your eye, and it's called retinoscopy. By examining the reflection of light off your retina, your eye care professional can determine if you are nearsighted, farsighted or have astigmatism. This is how they can also get a pretty good reading on the prescription required to correct your vision.
The main thing your doctor is checking for during this exam is how accurately your eyes can focus. When we use the retinoscope to shine light into your eye, a reddish light reflects off your retina, through your pupil. This is called the red reflex. We use the light to measure your focal length, or in layman's terms, it will determine the angle of refraction of light off your retina. And this is what tells us how well your eye is able to focus. And if we see that you aren't focusing properly, that's where the lenses come in. We hold a few prescription lenses in front of your eye to see which one will correct the refractive error. The lens power that works is the prescription you will need to rectify your impairment with glasses or contact lenses.
The retinoscopy exam is conducted in a dark room. You will usually be instructed to look at something behind the doctor. The exam doesn't include charts to be read, which means that a retinoscopy exam is also a really good way to accurately determine the prescriptions of those who may struggle with speech, like young children and the elderly.