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**Optics -- magnification of a converging lens**

**1. Homework Statement**

Dioptry of converging lens is $D=3$. What is magnification ##u##?

**2. Homework Equations**

**3. The Attempt at a Solution**

##\frac{1}{f}=D## - dioptry.

[tex]\frac{1}{f}=\frac{1}{p}+\frac{1}{l}[/tex]

[tex]u=\frac{l}{p}[/tex]

[tex]l=up[/tex]

[tex]D=\frac{l+p}{lp}=\frac{p(u+1)}{up^2}[/tex]

Putting ##d=0.25m##, I get

[tex]0.25\cdot 3\cdot u=u+1[/tex]

and ##u=-4##. Where I make a mistake?