Paranoid Intern - When to Leave the Office?

Hey everyone.

So I'm an intern at a local accounting company - it's not an IB, nothing finance related in the slightest, actually. My job requires me to work 7.5 hours a day, and as an hourly-paid intern, that is all I get paid for. So, usually, I arrive at 7:30 AM and leave at 3:45 - 4:15 PM. I eat lunch at my desk, so I'm pretty much working 8.5 hours on average a day. I put my time in, and I get my work done to the best of my ability, but I feel like I'm not getting enough "face time." The boss gets in at 9:00 AM and doesn't leave till maybe 5:30 PM, so he's putting in the same amount of hours as I do, and lots of other people at the company leave at 3:45 - 4:15 also, but I feel like I'm making a bad impression by doing so by leaving before he is. Am I over-reacting? If I've got all my work done it just seems silly to sit at my desk for no reason just to be at the office, but I'm not sure. Thanks.

As a side note, the company is super casual. People can wear shorts to work, and people are allowed to work from home for 3 out of the 5 days of the week.