The focal length of a convex lens is $30 \ cm$ and the size of the image is a quarter of the object,then the object distance is ........ $cm$.

The radius of curvature of the curved face of a plano-convex lens is $12 \,cm$ and its refractive index is $1.5$. Then,the focal length of the lens is (in $\,cm$)

When blue light is used instead of red light in a convex lens,its focal length will ......... .

$A$ double convex lens has two surfaces of equal radii $15 \, cm$ and refractive index $\mu = 1.5$. Its focal length is equal to ..... $cm$.

The two surfaces of a concave lens, made of glass of refractive index $1.5$, have the same radii of curvature $R$. It is now immersed in a medium of refractive index $1.75$. Then the lens:

$A$ convex lens has a power of $+5.0 \, D$ in air,where the refractive index of the lens material is $_a\mu_g = 1.5$. In a liquid of what refractive index should it be immersed so that it acts as a concave lens of focal length $100 \, cm$?