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A real image, same as the size of the object is formed $18 cm$ from a lens. Focal length of the lens is ____ $cm$ .

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Detailed Solution

A real image, the size of the object is formed

18 cm

from a lens. Focal length of the lens is

10 cm

.
Since the image is real, the lens must be convex. Also the image is the same size as the object, therefore the object must be at the radius of curvature.
It has been given that,
Object distance,

(u) = - 18 cm

Image distance

(v) = - 18 cm

Therefore, Radius of curvature

(R) = 18 cm

(Since the lens is convex, the radius of curvature is

+ ve

\Rightarrow f = \frac{R}{2}

\Rightarrow f = \frac{18}{2}

\Rightarrow f = 9 cm

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