15. The lens is located on the optical bench between the lamp and the screen, the screen gives a sharp increase in the light bulb. When the lens is moved 40 cm closer to the screen, it appeared sharp decrease of the light bulb. The focal length f of the lens, if the distance from the light bulb to the screen is 80 cm.

Detailed solution. Decorated in Microsoft Word 2003. (Target decided to use formula editor)