Sometimes, especially when performing an eye exam on a small child the eye doctor will focus a beam of light in the eyes. But why? Firstly, this test is a retinoscopy examination, and if you have issues with accurate vision, this is a preliminary way the eye doctor could assess it. By looking at the reflection of light off your retina, your eye care professional can decide 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 looking for during this exam is how accurately your eyes can focus on the light. We shine light into your eye because we are looking for what's known as the red reflex. The retinoscope sends light into your eye, and a red or orange light reflects through your pupil and off your retina. The angle at which the retinoscope's light refracts off your retina, which is what eye care professionals call your focal length, is precisely what tells us how well your eye can focus. If it becomes obvious that you are not focusing correctly, we hold up a variety of lenses with varying prescriptions in front of your eye to see which one corrects your vision. This is precisely how we find out what prescription your glasses or contact lenses need to be.
The eye doctor will perform your exam in a dark room. To make your eyes easier to examine, you'll generally be instructed to keep your eyes fixed on an object 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.