Select the incorrect order.

$A$ ray of light is incident on an equilateral glass prism placed on a horizontal table. For minimum deviation,which of the following is true?

The insecticide obtained from the neem plant is:

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:

$A$: The $O-O$ bond length in $H_2O_2$ is smaller than that in $O_2F_2$.
$R$: $H_2O_2$ is an ionic compound.

$A$ plano-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If the lenses are made of different materials with refractive indices $\mu_1$ and $\mu_2$ and $R$ is the radius of curvature of the curved surface of the lenses,then the focal length of the combination is: