There are various tests that you may have noticed at an eye exam and questioned what they are for. Having a bright light shined into your eyes could be one of them. Such as test is used to help determine the refractive error of your eye, and it's called retinoscopy. It sounds fascinating, but by examining the reflection of light off your retina, your eye doctor can determine whether you are nearsighted, farsighted or have astigmatism, and can also measure the prescription required to correct your vision.
In short, what we are looking for during the retinoscopy exam is checking how well your eye can focus. 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. We use the light to measure your focal length, or in other words, to calculate the angle of refraction of light off your retina. And this is what lets us know how well your eye is able to focus. If it becomes clear that you aren't focusing properly, that's where the lenses come in. We hold up a number of lenses with varying prescriptions in front of the eye to see which one fixes the refractive error. That lens power is the prescription you require to correct your sight with glasses or contact lenses.
The optometrist will run your exam in a darkened room. To make your eyes easier to examine, you'll usually be told to focus on an object behind the doctor. Because a retinoscopy exam doesn't involve any eye charts, it's also a particularly useful way to determine an accurate prescription for children or patients who have difficulty with speech.