Hope that you had a great holiday week! Welcome back to new week of idiom fun. ;)
Often a light bulb over the head of a cartoon character is used to indicate that he or she had suddenly come up with an idea!
And when people have a sudden realization about something or a new and exciting idea, it might be referred to as A Light Bulb Moment.
For example ... Billy was very sad after he was unable to sell the baskets of lemons he had. Then, he had a light bulb moment. "I know what I'll do! I'll open a lemonade stand."