On occasion, particularly when doing an eye exam on small children the eye doctor will focus a light in the eyes. But what does this do? Such as test is used to help test the refractive error of your eye, and it's called a retinoscopy exam. It sounds fascinating, but by looking at the way light reflects off your retina, the eye doctor can assess if you are nearsighted, farsighted or have astigmatism, and can also get a pretty good reading on the prescription you would need to correct your vision.
How well your eyes focus under the circumstance we create during the retinoscopy exam is really what we're looking for. When light shines into your eye using a retinoscope, a reddish light reflects off your retina, through your pupil. This is called the red reflex. We use the light to determine your focal length, or in other words, to calculate the precise angle at which light refracts off your retina which lets us know how well your eye focuses. And if it's apparent that you are not focusing correctly, we hold a few lenses with varying prescriptions in front of your eye to determine which one corrects the error. And that is precisely how we find out the prescription your glasses or contact lenses need to be.
All this happens in a dark room. You will usually be asked to focus on something behind the doctor. Not having to read any eye charts means that a retinoscopy exam is also a really useful tool to determine the prescriptions of the speech-impaired, or young children.