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Question: 

An achromatic lens is made of two very thin lenses, placed in contact, that have focal lengths $f_1 = -27.8 \textrm{ cm}$ and $f_2 = +25.3 \textrm{ cm}$.

  1. Is the combination converging or diverging?
  2. What is the net focal length?

Source: Giancoli, Douglas C., Physics: Principles with Applications, 7th Edition, 2014.

Quick Answer: 
  1. converging
  2. 281 cm

Giancoli 7th Edition, Chapter 25, Problem 51

(4:14)

Chapter 25, Problem 51 is solved.

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