“Finally/suddenly seeing the light” is another way to describe a “penny-dropping moment.”
(example usage of “I suddenly saw the light!” next to “The penny dropped!” from ‘Young Children Learning Through Schemas: Deepening the dialogue about learning in the home and in the nursery’ by Katey Mairs, The Pen Green Team, via ‘Google Books’)
see the light
Fig. to understand something clearly at last.
“After a lot of studying and asking many questions, I finally saw the light.”
“I know that geometry is difficult. Keep working at it. You'll see the light pretty soon.”
(from ‘McGraw-Hill Dictionary of American Idioms and Phrasal Verbs’ via ‘The Free Dictionary by Farlex’)
see the light
1. to understand something clearly, especially after you have been confused about it for a long time
“Sarah used to have very racist views, but I think she's finally seen the light.”
(from ‘Cambridge Idioms Dictionary, 2nd ed.’, also via ‘The Free Dictionary by Farlex, lower on the same page linked above)