Can you solve this riddle?

In a certain prison, there was 50 prisoners and 1 jailer. One day, the jailer gathered the 50 prisoners into a large room. He then said to the prisoners:
'I will give you a chance to be let out from this prison. Every day I will pick one of you at random and take you to a certain room. In that room is a light switch. When you are in that room you can either switch the light on, switch it off or just leave it as it is. Then I will return you to your cell. Now, here is what you guys need to do. When you think all of you have turned the light on at least once, one of you must tell me "All of us have turned the light on at least once". If you are right, I will let you out. If you're wrong , I will kill all the prisoners. Today is the only day you guys can communicate with each other. From tomorrow onwards none of you will be able to communicate to one another. I will not touch the switch under any circumstances. Now, I will leave you here to discuss."
A couple of years later, the prisoners all walked out of the prison happily.

What was their solution to the jailer's problem?

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