system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do very young or verbally challenged children get fitted for glasses if they are unable to verbally indicate that their vision is much clearer?
gpt
With normal vision, an object from the world should focus to tight spot on the back of the eye, known as the retina. One way to check this is to shine a beam of light and see how big of a spot it is on the retina. Someone with bad but correctable vision would have a large spot rather than a tight one.

By adjusting the properties of the beam, you can modify it so that it bends similarly to how it would go through a glasses' lens. By figuring out the necessary changes in the beam to make a tight spot, it's possible to recover the lens refractive power needed to correct their eyes!

This method is more complicated and expensive than just swapping lenses and asking a if it's clearer. But it's definitely worth correcting major errors in a baby or young child's vision or else their brain will stop using the nerves from the eye, causing irreversible vision loss.