There may be a few tests that you have seen during an eye exam and wondered how they work. Having beams of light shined into your eye could be an example. Firstly, this test is known as a retinoscopy examination, which is a preliminary way to assess the refractive error of your eye. Whether you're near or farsighted, or you have astigmatism, examining the way light reflects off your retina is a test your optometrist can use to determine whether you need vision correction.
How well your eyes are able to focus during the retinoscopy exam is really what we're looking for. When light shines into your eye using a retinoscope, a reddish orange light reflects off your retina, through your pupil. We call this the red reflex. We use the light to determine your focal length, or in simpler words, to measure the precise angle of refraction of light off your retina which lets us know how well your eye is able to focus. If it becomes obvious that you can't focus correctly, that's where the lenses come in. We hold up a variety of prescription lenses in front of your eye to see which one fixes your vision. That lens power is the prescription you will need to rectify your vision with glasses or contact lenses.
All this happens in a dark or dimmed room. To make your eyes easier to examine, you'll usually be told to look at an object 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.