HC Verma Chapter 18 Problem 56 – Virtual object for thin lens

Given

Converging beam aimed at point 20 cm behind lens; f=30 cm lens intercepts beam

Find

Image distance

Formula

u = +20 cm (virtual object); 1/v = 1/f + 1/u

Solution

1/v = 1/30 + 1/20 = 5/60; v = 12 cm (real image)

Answer

v = 12 cm

Diagram / Concept

Virtual object: beam converging to a point behind lens; lens focuses it sooner

Notes

Result is real image at 12 cm from lens