A convex lens of focal length 20cm


A convex lens of focal length $20 \mathrm{~cm}$ and a concave lens of focal length $10 \mathrm{~cm}$ are placed $10 \mathrm{~cm}$ apart with their principal axis coinciding. A beam of light travelling parallel to the principal axis and having a beam diameter $5 \mathrm{~mm}$, is incident on the combination. Show that the emergent beam is parallel to the incident one. Find the beam diameter of the emergent beam.


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