Jacob Riis was a Danish-American social reformer, journalist, and photographer born in 1849. He is best known for his work in the late 19th century, where he documented the living conditions of the poor in New York City. His book, "How the Other Half Lives," published in 1890, highlighted the struggles of immigrants and the urban poor, using both text and photographs to raise awareness about poverty. Riis's efforts played a significant role in the progressive movement, influencing public policy and social reform. His work helped to inspire changes in housing, sanitation, and labor laws, making him a key figure in the fight for social justice during the Gilded Age.