As shown in the figure,a concave lens of focal length $50 \, cm$ and a convex lens of focal length $25 \, cm$ are placed in contact with each other. If a parallel beam of light is incident on the system,how will it emerge?

The focal length of a lens of dispersive power $0.45$ which should be placed in contact with a convex lens of focal length $84 \,cm$ and dispersive power $0.21$ to make the achromatic combination from the two lenses,in $cm$ is

Two convex lenses of focal length $20\,cm$ each are placed coaxially with a separation of $60\,cm$ between them. The image of the distant object formed by the combination is at $...........\,cm$ from the first lens.

It is desired to make an achromatic combination of two lenses ($L_1$ and $L_2$) made of materials having dispersive powers $\omega_1$ and $\omega_2$ $(< \omega_1)$. If the combination of lenses is converging,then:

Two thin plano-convex lenses each of focal length $f$ are placed as shown in the figure. The ratio of their effective focal lengths in the three cases is

$A$ bi-convex lens is formed with two thin plano-convex lenses as shown in the figure. The refractive index $n$ of the first lens is $1.5$ and that of the second lens is $1.2$. Both the curved surfaces are of the same radius of curvature $R = 14 \, cm$. For this bi-convex lens,for an object distance of $40 \, cm$,the image distance will be.......$cm$