Public domain photos

The image shows a diagram of a ray with a point at the center and a line at the end of it, along with some text written on a white background. The text reads "diagram of a lens with a focal length of b1, b2, b3, b4, b5, b6, b7, b8, b9, b10, b11, b12, b13, b14, b15, b16, b17, b18, b19, b20, b21, b22, b23, b24, b25, b26, b27, b28, b29, b30, b31, b32, b33, b34, b35, b36, b37, b38, b39, b40, b41, b42, b43, b44, b45, b46, b47, b48, b50, b51, b52, b53, b54, b55, b56, b57, b58, b59, b60, b61, b62, b63, b64, b65, b66, b67, b68, b69, b70, b71, b72, b73, b74, b75, b76, b77, b78, b79, b80, b81, b82, b83, b84, b85, b86, b87, b90, b91, b92, b93, b94, b95, b96, b97, b98, b99, b100, b110, b101, b102, b103, b104, b105, b106, b107, b112, b113, b114, b115, b130, b131, b132, b129, b134, b135, b136, b137, b138, b139, b140, b150, b170, b175, b176, b180, b190, b200, b205, b220, b230, b225, b250, b260, b270, b265, b267, b268, b269, b290, b295, b320, b300, b340, b350, b380, b400, b450, b470, b480, b500, b550, b460, b600, b650, b700, b750, b800, b900, b1.