$A$ 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:

Two thin lenses of focal length $f_{1}$ and $f_{2}$ are in contact and coaxial. The power of the combination is . . . . . . .

$A$ thin convex lens of focal length $10 \ cm$ is placed in contact with a concave lens of the same material and of the same focal length. The focal length of the combination will be:

Two identical glass $(\mu_g = 3/2)$ equiconvex lenses of focal length $f$ each are kept in contact. The space between the two lenses is filled with water $(\mu_w = 4/3).$ The focal length of the combination is

$A$ convex lens of power $+6 D$ is placed in contact with a concave lens of power $-4 D$. What is the nature and focal length of the combination?

$A$ 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 curved surfaces have 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$