This is "An Old Fiddler" by Frans van Mieris the Elder. Painted in the Dutch north, it's a classic example of atronie, a genre of painting popular in 17th-century Holland. These paintings often featured comical stock characters with exaggerated facial expressions.
You can see that the old fiddler is dressed in a fashion from almost a century before he was painted. His white ruff and cuff, puffed sleeve, and curve-brimmed hat are all characteristic of an earlier style. The violin on the windowsill helps create the illusion of space, projecting outwards towards us.
The figure behind him, a woman writing on a slate, also adds depth to the scene. Her shadowy presence creates a sense of atmosphere, as if she's keeping track of the old man's tab. Despite his relaxed pose, there's something troubled about this character - perhaps that's what makes the painting so intriguing.
It's worth noting that this is likely a copy made in the 18th century, rather than the original from 1660. The ultra-smooth finish on the oak panel gives it a greater illusion of reality, allowing attention to tiny details like the snail and the gathers in his neck ruff.
An Old Fiddler hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.