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# The refractive index of a material of a plano concave lens is $\large\frac{5}{3}$, the radius of curvature is $0.3\;m$. The focal length of the lens in air is

$\begin {array} {1 1} (1)\;-0.45\;m & \quad (2)\;-0.6\;m \\ (3)\;-0.75\;m & \quad (4)\;-1.0\;m \end {array}$

(1) -0.45 m