There are some tests that you have seen at an eye exam and asked yourself what they measure. Having beams of light shined into your eye could be one of them. Firstly, this test is a retinoscopy examination, and it's a basic way to determine the refractive error of your eye. By merely examining the reflection of light off your retina, your eye doctor can decide whether you are nearsighted, farsighted or have astigmatism. This is how they may also get a pretty good reading on the prescription required to correct your vision.
In short, what we are looking for during a retinoscopy exam is checking how well your eye can focus. We shine light into your eye because we are looking for what's known as your red reflex. The retinoscope sends light into your eye, and a red or orange light reflects through your pupil and off your retina. The retinoscope measures your focal length, or in layman's terms, it will determine the angle at which light refracts off your retina. And this is what lets us know how well your eye is able to focus. And if we notice that you aren't focusing properly, we hold a variety of lenses with varying prescriptions in front of your eye to see which one rectifies the error. The lens power that works is the prescription you will need to rectify your sight with glasses or contact lenses.
The optometrist will perform your exam in a darkened room. To make your eyes easier to examine, you'll generally be asked to look at an object behind the doctor. Because a retinoscopy exam doesn't involve any eye charts, it's also a particularly useful way to determine an accurate prescription for children or patients who have difficulty with speech.