A thin converging lens of focal length $25$ cm is kept in contact with a thin diverging lens of focal length $20$ cm. Focal length of the combination is:-

+100 cm

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- 100 cm

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45 cm

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5 cm

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Solution

The correct option is B - 100 cm
$\begin{matrix} f_{1} = 25 c m f_{2} = - 20 c m Focal length of combination \frac{1}{f} = \frac{1}{f_{1}} + \frac{1}{f_{2}} \frac{1}{f} = \frac{1}{25} - \frac{1}{20} \frac{1}{f} = \frac{4 - 5}{100} f = - 100 c m . \end{matrix}$

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A thin converging lens of focal length 25cm is kept in contact with a thin diverging lens of focal length 20 cm. Focal length of the combination is:-

The correct option is B - 100 cmf1=25cmf2=−20 cm Focal length of combination 1f=1f1+1f21f=125−1201f=4−5100f=−100 cm.

A thin converging lens of focal length $25$ cm is kept in contact with a thin diverging lens of focal length $20$ cm. Focal length of the combination is:-

The correct option is B - 100 cm
$\begin{matrix} f_{1} = 25 c m f_{2} = - 20 c m Focal length of combination \frac{1}{f} = \frac{1}{f_{1}} + \frac{1}{f_{2}} \frac{1}{f} = \frac{1}{25} - \frac{1}{20} \frac{1}{f} = \frac{4 - 5}{100} f = - 100 c m . \end{matrix}$