Geometrical Optics

behaves as concave lens if $\left|f_{\text {convex }}\right|>\left|f_{\text {concave }}\right|$

behaves as concave lens if $\left|f_{\text {convex }}\right|<\left|f_{\text {concave }}\right|$

behaves as convex lens if $\left|f_{\text {convex }}\right|>\left|f_{\text {concave }}\right|$

Focal length of the lens system will change if the positions of two lenses are interchanged