On occasion, particularly when performing an eye exam on small children the eye doctor will shine a light in the eye. But why? Firstly, this test is known as a retinoscopy examination, which is a preliminary way to determine the refractive error of your eye. Whether you're near or farsighted, or you have astigmatism, examining the reflection of light off your retina is a way your optometrist is able to see if you need vision correction.
Basically, what we are looking for during the retinoscopy exam is checking how well your eye can focus. We shine light into your eye because we are looking for what's known as your red reflex. The retinoscope aims light into your eye, and a reddish light reflects through your pupil and off your retina. We use the light to measure your focal length, or in other words, to measure the angle at which light refracts off your retina which lets us know how well your eye focuses. If it becomes obvious that you can't focus properly, we hold different lenses with varying prescriptions in front of your eye to see which one will correct the refractive error. And that is exactly how we find out what prescription your glasses or contact lenses need to be.
The optometrist will run your exam in a darkened room. To make your eyes easier to examine, you'll generally be asked to focus on something behind the doctor. The exam doesn't include charts to be read, which 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.