39.Two converging lenses with focal length f and 2f are positioned at 3f distance apart.A parallel beam of light is incident on the lens with focal length f.If d is the incident beam width,what is the width of the emerging beam?

Q. Two converging lenses with focal lengths f and 2f are positioned at a distance 3f apart. A parallel beam of light is incident on the lens with focal length f. If d is the incident beam width, what is the width of the emerging beam?

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 $20cm$ and a concave lens of focal length $f$ are mounted coaxially $5cm$ 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 20 cm and a concave lens of focal length 10 cm are placed 10 cm apart with their principal axes coinciding. A beam of light travelling parallel to the principal axis and having a beam diameter 5.0 mm, is incident on the combination. Show that the emergent beam is parallel to the incident one. Find the beam diameter of the emergent beam.

Q. 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.