Sometimes, particularly when performing an eye exam on a small child the optometrist will direct a light in the eyes. But why? Firstly, this test is known as a retinoscopy examination, and if you struggle with accurate vision, this is one way the optometrist could assess it. It sounds fascinating, but by looking at the reflection of light off your retina, your optometrist can decide whether 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.
The most important thing an eye doctor is looking for during this exam is how well your eyes can focus. We begin the exam by looking for what's known as your red reflex. The retinoscope sends a beam of 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, also called your focal length, is the thing that tells us how well your eye can focus. If it becomes obvious that you can't focus properly, that's when we use a set of lenses. We hold different lenses with varying prescriptions in front of the eye to see which one rectifies the error. That lens power is the prescription you will need to correct your vision with glasses or contact lenses.
All this happens in a dark room. You will usually be told to look at something behind the doctor. Because a patient doesn't need to read eye charts during a retinoscopy exam, it's also a particularly useful way to determine an accurate prescription for children or patients who have difficulty with speech.