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Physics · Grades 11–12

Image distance for a converging lens

This model uses a thin lens, a real object beyond the focal point, and positive real-image distances.

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Rule and assumptions

This model uses a thin lens, a real object beyond the focal point, and positive real-image distances.

1/f = 1/do + 1/di

Worked examples

A thin converging lens has focal length +2 cm. A real object is 63 cm from it, beyond the focal point. Using 1/f = 1/do + 1/di, find real image distance. Round to 3 decimal places if needed. Answer in cm.

1/f = 1/do + 1/di (2 × 63) / (63 − 2) = 2.066

2.066 cm

A thin converging lens has focal length +27 cm. A real object is 65 cm from it, beyond the focal point. Using 1/f = 1/do + 1/di, find real image distance. Round to 3 decimal places if needed. Answer in cm.

1/f = 1/do + 1/di (27 × 65) / (65 − 27) = 46.184

46.184 cm

Try two questions

  1. A thin converging lens has focal length +13 cm. A real object is 26 cm from it, beyond the focal point. Using 1/f = 1/do + 1/di, find real image distance. Round to 3 decimal places if needed. Answer in cm.

  2. A thin converging lens has focal length +14 cm. A real object is 71 cm from it, beyond the focal point. Using 1/f = 1/do + 1/di, find real image distance. Round to 3 decimal places if needed. Answer in cm.

Answers and reasoning

Answers and marking notes
  1. 26 cm

    1/f = 1/do + 1/di (13 × 26) / (26 − 13) = 26

  2. 17.439 cm

    1/f = 1/do + 1/di (14 × 71) / (71 − 14) = 17.439

Further reading and review references

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