Find the focal length of the lens if the object is at a distance of 6 cm from it, the distance to a clear image

Find the focal length of the lens if the object is at a distance of 6 cm from it, the distance to a clear image on the screen is 12 cm in the answer, explain What is focus and focal length.

Let’s turn to the thin lens formula:

1 / F = 1 / L1 + 1 / L2, where F is the focal length, L1 and L2 are the distances from the object to the lens and from the image to the lens.

1 / F = 1/12 + 1/6 = 3/12 = 1/4;

F = 4 cm.

Answer: the sought focal length was 4 cm. The focus of the lens is the point through which any ray will pass after passing through the lens (for a diffusing lens, this distance is imaginary), respectively, the focal length is the distance from the lens to the focus.