Is it normal to have a sexual fantasy about a girl i have a crush on

There's this girl in my class at school. I sometimes fantasize about her because she is so pretty. I have a crush on her but I only watch her from afar. One day, I'll get enough guts to talk to her. Anyway, the other night, I was lying in bed and I started dreaming about her being a damsel in distress. I dreamt that some evil sultan put a gold collar around her neck and chained her to his throne making her a slave girl. Most importantly of all: in the dream, before the sultan enslaved her, he made her take her clothes off.
I've never seen her naked but I sometimes wonder what she looks like naked. Is this normal.