Question

The image obtained with a convex lens is erect and it's length four times the length of the object. If the focal length of the lens is 20 cm .calculate the object and image distance

Answer 1

$\bf{\underline{Question:-}}$

The image obtained with a convex lens is erect and it's length four times the length of the object. If the focal length of the lens is 20 cm .calculate the object and image distance.

$\bf{\underline{Given:-}}$

• image obtained with a convex lens is erect.
• it's length four times the length of the object.
• . If the focal length of the lens is 20 cm .

$\bf{\underline{Find:-}}$

• calculate the object and image distance.

$\bf{\underline{Solution:-}}$

$→ \sf\large M=\frac{v}{u} = +4 = \frac{ h'}{h}$

→ v = 4 u

$\sf → \frac{1}{f} = \frac{1}{v} - \frac{1}{u}$

$\sf → \frac{1}{20} = \frac{1}{4u } - \frac{1}{u}$

$\sf → \frac{-3}{4u}$

$\sf→ \frac{4u}{3} = -20 cm$

→ u = -15 cm

so

v = - 60 cm

• so this is a virtual image.

$\bf{\underline{Hence:-}}$

• image size = 4 times object size.

Answer 2

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