Many people correctly pointed out that such imagery trivializes the purpose of protest and the professional role of the police, images that could dangerously alter the opinions of impressionable young people.
Can anybody clarify the use of the comma in the sentence above?
For me, I feel it is reasonable and acceptable but I would like to comprehend the grammatical justification for that.
The clause, "images that could dangerously alter the opinions of impressionable young people" is an aside that the author thinks is important to consider within the context of the same sentence.
At the same time, the main part of the sentence can stand alone just fine without the information, too, which is why the author had written this as an aside ("Parenthetical phrases: Aside," Wikipedia).
In the meantime, why did the author choose a comma over parentheses or dashes?
Consider this:
"Many people correctly pointed out that such imagery trivializes the purpose of protest and the professional role of the police (images that could dangerously alter the opinions of impressionable young people)."
or this:
"Many people correctly pointed out that such imagery trivializes the purpose of protest and the professional role of the police -- images that could dangerously alter the opinions of impressionable young people."
When used as an aside, all three could be used to do the same thing (i.e., a comma, parentheses and dashes).
Which we choose depends on much impact we want to make with the aside.
If you read the examples above (including the original), you'll see that dashes generally make the most impact because they interrupt the flow of the reader, almost telling the reader to stop and pay attention to the words within the dashes.
Meanwhile, commas make the the least impact because they are a part of the sentence.
And parentheses, are simultaneously somewhat removed from the rest of sentence without interrupting the flow of the sentence completely.
You can take a look at this article and metaphor from Grammar Girl for more information: Parentheses are the quiet whisper of an aside, commas are the conversational voice of a friend walking by your desk, and dashes are the yowl of a pirate dashing into a fray.
But as she also says, the important thing to remember that this is not one hundred percent cut-and-dry and as you read more, you'll start to figure out which one you want to use and when.
