Why the focal length of convex lens is always positive?
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Solution
The reason convex lenses always have positive focal length.
The parallel rays of light when passed through the convex lens it converges the rays on the right side, thus, the focus is on the right side of the lens.
As the rays are converged toward the right side of the lens, according to the sign convention of lenses, the focal length is measured in the direction of the incident ray, thus, the focal length should be positive.
