As a rule, it’s probably best to avoid “random” internet sources on matters of how light works because there’s so much confidently parroted misinformation out there. For example, this is completely wrong: youtu.be/FAivtXJOsiI See here for correct answers to that issue: youtu.be/CiHN0ZWE5bk

For how mirrors work see this: scientificamerican.com/…/what-is-the-physical-pro… youtu.be/rYLzxcU6ROM