$A$ convex lens of focal length $84 \ cm$ is in contact with a concave lens of focal length $12 \ cm$. The power of the combination (in diopters) is:

$A$ convex lens of focal length $100 \, cm$ and a concave lens of focal length $10 \, cm$ are placed coaxially at a separation of $90 \, cm$. If a parallel beam of light is incident on the convex lens,then after passing through the two lenses,the beam:

As shown in the figure, a combination of a thin plano-concave lens and a thin plano-convex lens is used to image an object placed at infinity. The radius of curvature of both the lenses is $30\,cm$ and the refractive index of the material for both the lenses is $1.75$. Both the lenses are placed at a distance of $40\,cm$ from each other. Due to the combination, the image of the object is formed at a distance $x = ..........cm$ from the concave lens.

Two thin lenses whose powers are $+2 \ D$ and $-4 \ D$ respectively are combined. The power of the combination is:

$A$ convex lens is in contact with a concave lens. The magnitude of the ratio of their focal lengths is $2/3$. Their equivalent focal length is $30 \, cm$. Their individual focal lengths in $cm$ will be

Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The combination is equivalent to a single lens of power: