Question

A convex lense of focal length 30cm forms an image on other side of the lense at a distance of 90cm what is the object distance

Answer 1

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Given :

• Focal Length (f) = 30 cm
• Image Distance (v) = 90 cm
• Convex Lens

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To Find :

• Object Distance (u)

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Solution :

Use lens formula :

$\large \star {\boxed{\boxed{\sf{\dfrac{1}{f} \: = \: \dfrac{1}{v} \: - \: \dfrac{1}{u}}}}} \\ \\ \implies {\sf{\dfrac{1}{u} \: = \: \dfrac{1}{v} \: - \: \dfrac{1}{f}}} \\ \\ \implies {\sf{\dfrac{1}{v} \: = \: \dfrac{1}{90} \: - \: \dfrac{1}{30}}} \\ \\ \implies {\sf{\dfrac{1}{u} \: = \: \dfrac{1 \: - \: 3}{90}}} \\ \\ \implies {\sf{\dfrac{1}{u} \: = \: \dfrac{-2}{90}}} \\ \\ \implies {\sf{\dfrac{1}{u} \: = \: \dfrac{-1}{45}}} \\ \\ \implies {\sf{u \: = \: - \: 45}} \\ \\ \\ {\underline{\sf{\therefore \: \: Object \: Distance \: (u) \: = \: -45 \: cm}}}$