Physic Labs

Problem 3

A thin converging lens of focal length f=20f=20 cm stands in air. A small luminous object of height H=2.0H=2.0 cm is perpendicular to the principal axis, u=30u=30 cm in front of the lens. Use the convention that a real image has v>0v>0, the lens equation 1/f=1/u+1/v1/f=1/u+1/v, and lateral magnification m=−v/um=-v/u. (a) Find the image distance and height. (b) State whether the image is real or virtual and upright or inverted. (c) The object is moved farther away to u=60u=60 cm; find the new image and describe the size change.
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u=60 cm:1v=120−160=130,v=30 cmu=60\ \mathrm{cm}:\quad \frac1v=\frac1{20}-\frac1{60}=\frac1{30},\quad v=30\ \mathrm{cm}
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Insert the new object distance into the same equation; the real image lies 30 cm on the other side. Since the object distance remains greater than the focal length, a real image is expected.