First slide

Ray optics

Question

A person has near point at 60 cm. The focal length of spectacle lenses to read at 22 cm having glasses separated 2 cm from the eyes, is :

Difficult

Solution

$\begin{array}{l} v = - 60 c m, u = - (22 - 2) - - 20 c m, f = ? \\ \frac{1}{v} - \frac{1}{u} = \frac{1}{f} \\ \frac{1}{- 60} - \frac{1}{- 20} = \frac{1}{f} \\ \frac{1}{20} - \frac{1}{60} = \frac{1}{f} \\ \frac{2}{60} = \frac{1}{f} \Rightarrow f = 30 c m \end{array}$

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