This is the Portrait of the Artist's Daughter Mary by Thomas Gainsborough. Painted in 1777, it's one of two portraits made around this time of the artist's daughters, Mary and Margaret. What you see here is a painting that's quite unique because Gainsborough has signed it and inscribed the sitter's name on the work itself.
You'll notice how loose and experimental the handling is in this painting. This wasn't an unusual approach for Gainsborough, as we can see in some other works featuring his daughters, such as chasing a butterfly or playing with a cat. These paintings reflect just how close he was to his family members.
One of the most striking things about this portrait is its relationship to another work from earlier generations. You might be able to pick up on hints of Peter Paul Rubens' style in Gainsborough's depiction of Mary. It shows just how much influence Gainsborough drew from great artists who came before him.
Portrait of the Artist's Daughter Mary 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.