system
Given the following post, respond as if you're commenting on Reddit:
human
If a concave lens and mirror are kept under water, the focal length of the mirror stays same but changes for the lens. Why is this so? Isn't the physical length the same no matter the medium?
gpt
Focal length isn't a physical length, though. For a lens, it is a function of the change in the index of refraction between media, because a lens works by refracting light. If the index of refraction is the same, then there's no refraction at all and the lens doesn't function. Which you can see if you put a glass lens in water and try to look through it. 

A mirror does not function by refraction, so the index of refraction of surrounding media doesn't change anything about it.