An eye-opener

An eye-opener: aquilo que chama sua atenção para algo em que você não tinha reparado antes / algo que faz você "abrir os olhos", algo que deixa você de "olhos bem abertos"

“Al Gore's movie "An Inconvenient Truth" was a real eye-opener for me. I hadn't really understood how serious global warming was until I saw it.”

“Cathy said her trip to Thailand was quite an eye-opener for her. She'd never realized before how beautiful Thailand was, or how friendly the people were.” (www.englishclub.com)