Question

Why is focal length of convex lens positive and concave is negetive

Answer 1

Now the convex lens converges the incident rays. ... you can easily verify that the focal length is in the positive x direction for a convex lens while it is along the negative x direction for a concave lens. So f is positive for a convex lens and negative for a concave lens.

Answer 2

Answer: -

Now the convex lens converges the incident rays. ... you can easily verify that the focal length is in the positive x direction for a convex lens while it is along the negative x direction for a concave lens. So f is positive for a convex lens and negative for a concave lens. The reason is the sign altering

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