On the axis of the collecting lens at a distance a = 25 cm from it there is a very small light source. On the other side of the lens, once at a distance l1 = 27 cm, and another time at a distance l2 = 48 cm, a screen is placed. The illumination of the light spot on the screen is the same in both cases. Determine the focal length of the lens.
Detailed solution. Format gif
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