Sometimes, particularly when performing an eye exam on a small child the optometrist will shine a light in the eyes. But why? Such as test is used to help test the refractive error of your eye, and it's known as a retinoscopy exam. By looking at the way light reflects off your retina, your eye care professional can assess whether you are nearsighted, farsighted or have astigmatism, and can also measure 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 we use the retinoscope to shine light into your eye, a reddish orange light reflects off your retina, through your pupil. This is known as the red reflex. The angle at which the retinoscope's light refracts off your retina, also called your focal length, is the thing that lets us know how well your eye can focus. If it becomes obvious that you aren't focusing correctly, that's when we use a set of lenses. We hold different prescription lenses in front of your eye to see which one rectifies your vision. The lens power that works is the prescription you require to rectify your impairment with glasses or contact lenses.
All this happens in a darkened room. You will usually be instructed to look at something behind the doctor. This makes eyes easier to examine. Unlike other eye exams, your doctor won't ask you to read letters off charts. This means that a retinoscopy exam is also a really useful tool to determine the prescriptions of those who may struggle with speech, like young children and the elderly.