This is "A Bearded Man holding a Lamp" by Guercino, painted in the Italian north. You're looking at a copy of one of his most famous works, "Susannah and the Elders". That painting tells the story of Susannah, who's being spied upon while bathing by two old men who are threatening to accuse her of adultery.
You can see the same scene here, but with a slight twist. In Guercino's original work, the old man holds a staff, whereas in this copy he's holding a lamp instead. This was likely done for a specific reason or commission, as we don't know who exactly painted this version.
It's thought to be an Italian artist's work, and given that Guercino's original wasn't sent to Spain until 1664, it's possible this painting was made before then. We're not entirely sure, but one thing is certain: you're looking at a copy of a masterpiece by a renowned Baroque painter.
A Bearded Man holding a Lamp 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.