Question

A convex lens forms an image on a brick wall 3 m from the lens, of the filament of a headlight lamp 10 cm from the lens.  What is the focal length of the lens and what is the size of the image as seen at the brick wall if the actual size of the filament is 10 mm ?

Answer 1

object distance s = 10 cm

image distacne s' = 3m = 300m

focal length f = ?

1/s + 1/s' = 1/f

1/10 + 1/300 = 1/f

focal length f = 9.7 cm <<<<<--------ANSWER

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magnification m = s'/s = y'/y

image height y' = y*s'/s

y' = 10*300/10

y' = 300 m <<<<<--------ANSWER