A convex lens of focal length 15 centimeters is placed in contact with a concave lens of focal length 25 centimeters. What is the net focal length of the combined lens system?

💡 Solution & Explanation

The combined focal length of two thin lenses in contact is determined by summing their individual optical powers, where power is the reciprocal of focal length with proper sign conventions. The convex lens has a positive focal length of 15 centimeters and the concave lens has a negative focal length of 25 centimeters, yielding a net optical power of one over fifteen minus one over twenty-five, which equals positive two over seventy-five diopters. Taking the reciprocal gives a net focal length of positive 75 centimeters. Incorrect signs lead to wrong magnitude or direction.