Sometimes, especially when performing an eye exam on a small child the optometrist will focus a beam of light in the eye. But what does this do? This test is known as a retinoscopy examination, and if you have issues with accurate vision, this is one way the eye doctor could assess it. By looking at the reflection of light off your retina, the eye care professional can assess whether 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 well your eyes can focus. We shine light into your eye because we are looking for what we call the red reflex. The retinoscope aims 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 measure the angle of refraction of light off your retina which tells us how well your eye is able to focus. If it becomes clear that you are not focusing properly, we hold up a number of prescription lenses in front of the eye to determine which one fixes the refractive error. And that is precisely how we find out what prescription your glasses or contact lenses need to be.
These exams are performed in a darkened room. You will usually be told to look at something ahead, just behind the doctor. Unlike other eye exams, your doctor won't ask you to read any charts. This 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.