A thin convex lens $\left(L_1\right)$ of focal length $80$cm and a thin concave lens$\left(L_2\right)$ of focal length f are kept co-axially, $20$cm apart as shown in figure. When a narrow and parallel beam of light is incident on the convex lens, beam emerging from the concave lens is also a parallel beam. Find f.

Q. A convex lens of focal length 20 cm and a concave lens of focal length $f$ are mounted coaxially 5 cm apart. Parallel beam of light incident on the convex lens emerges from the concave lens as a parallel beam. Then $f$ in cm is:

Q. A convex lens of focal length $1.0cm$ and a concave lens of focal length $0.25m$ are $0.75m$ apart. A parallel beam of light is incident on the convex lens. The beam emerging after refracting from both lenses is

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

Q. A convex lens of focal length $1metre$ and a concave lens of focal length $0.25cm$ are kept $0.75m$ apart. A parallel beam of light first passes through the convex lens, then through the concave lens and comes to a focus $0.5m$ away from the concave lens.

Q. A convex lens of focal length $20cm$ and a concave lens of focal length $10cm$ are placed $10cm$ apart with their principal axes coinciding. A beam of light travelling parallel to the principal axis and having a beam diameter $5.0mm$, is incident on the combination. Show that the emergent beam is parallel to the incident one. Find the beam diameter of the emergent beam.