Having had to learn English, rather than acquiring it in the course of my rearing (I've often wondered how that word came to be!), I have to differ from the opinions in the at/on debate.

The way I learned it, if the optics are scratched, i.e. the glass is scratched, then the scratch is on the optics, while, if the scratch is near the optics or on the optics, it can be said to be at the optics.

You can be at the intersection of Main street and Broadway, which implies that you're at one of the four corners of the intersection, or you can be on the intersection, meaning you're standing at the intersection of the centerlines, or you can be inthe intersection, meaning that you're no longer on the sidewalk, but are out in the roadway. Note the ironic appearance of the word "at."

I wonder why there's been no mention of the word in. The scratch can be in the optics as well. Both in and on are both more common usage in American English than at would be in this context.

The difference lies in that in and on are specific, while at is not so much so.

RE