# If a concave and convex lens of equal focus are kept in contact what is the effective focal length?

Effective focal length in

The powers of two lenses are simply added when they remain in contact.

For convex lenses, the power of the lens is positive; for concave lenses, it is negative.

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When a concave and convex lens of equal focus are kept in contact, the effective focal length is the arithmetic sum of their individual focal lengths.

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