We know the lens maker formula for thin lens is
#color (magenta)(1/f=(mu-1)(1/R_1-1/R_2))#
where
#fto# focal length.
#muto# refractive index of the medium of the lens w.r. to air.
#R_1andR_2# are two radii of curvature of two surfaces of the lens.
When the lens is convex
then #R_1=0.7m and R_2=-0.7m#
And #mu=1.8#
So by lens maker formula we get
#color (blue)(1/(f_"convex") =(mu-1)(1/R_1-1/R_2))#
#color (blue)(=>1/(f_"convex") =(1.8-1)(1/0.7+1/0.7))#
#color (blue)(=>1/(f_"convex") =0.8xx2/0.7#
#color (blue)(=>f_"convex" =7/16=0.4375m#
When the lens is concave
then #R_1=-0.7m and R_2=0.7m#
And #mu=1.8#
So by lens maker formula we get
#color (green)(1/(f_"concave") =(mu-1)(1/R_1-1/R_2))#
#color (green)(=>1/(f_"concave") =(1.8-1)(-1/0.7-1/0.7))#
#color (green)(=>1/(f_"concave") =-0.8xx2/0.7#
#color (green)(=>f_"concave" =-7/16=-0.4375m#