Calculating Object Distance from Lens with Magnification and Focal Length

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SUMMARY

The discussion centers on calculating the distance of an object from a lens with a focal length of -15 cm, given that the image formed is half the size of the object. Using the magnification formula m = -q/p, where m is the magnification, q is the image distance, and p is the object distance, the relationship q = -(1/2)p is established. By applying the lens formula 1/p + 1/q = 1/f, the object distance p is determined to be 15 cm. The solution is confirmed as correct.

PREREQUISITES
  • Understanding of lens formulas, specifically 1/p + 1/q = 1/f
  • Knowledge of magnification concepts in optics
  • Familiarity with the sign conventions for focal length and distances in lens equations
  • Basic algebra skills for manipulating equations
NEXT STEPS
  • Study the derivation of the lens formula 1/p + 1/q = 1/f
  • Explore different types of lenses and their focal lengths
  • Learn about real and virtual images in optics
  • Investigate the effects of changing object distance on image characteristics
USEFUL FOR

Students studying optics, physics educators, and anyone involved in optical engineering or lens design.

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Homework Statement


An object is in front of a lens with a focal length of -15cm. An image ½ the side of the object is formed. How far is the object from the lens?

Homework Equations


m - magnification
q - distance from image
p - distance from object
f - focal length

m = -q/p
1/p + 1/q = 1/f

The Attempt at a Solution


m = -q/p so q = -(1/2)p

1/p + 1/q = 1/f
1/p + 1/(-1/2p) = 1/-15cm
-1/p = 1/-15cm
p = 15 cm
 
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