A point light source is located on the main optical axis of a thin collecting lens at a distance a=30 cm from the lens. A bright spot is observed on the screen located perpendicular to the main optical axis at a distance l1=10 cm from the lens. The size of the spot does not change if the screen is placed at a distance of l2=20 cm from the lens. Determine the focal length of the lens.
Detailed solution. Format gif
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