An object is placed at $30 c m$ from a convex lens and its real image is formed at $20 c m$ away from the lens. Calculate the focal length of convex lens. Draw ray diagram also.

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Solution

Here,
Image distance $= v = 20 c m$ ,
Object distance $= u = - 30 c m$ ,
Lens equation is given by,
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$ ,
putting the values we get,
$\frac{1}{20} - \frac{1}{- 30} = \frac{1}{f}$ ,

$∴ f = 12 c m$

$∴$ Focal length of convex lens $= f = 12 c m$

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An object is placed at 30cm from a convex lens and its real image is formed at 20cm away from the lens. Calculate the focal length of convex lens. Draw ray diagram also.

Here,Image distance=v=20cm,Object distance=u=−30cm,Lens equation is given by,1v−1u=1f,putting the values we get,120−1−30=1f,∴f=12cm∴ Focal length of convex lens=f=12cm

An object is placed at $30 c m$ from a convex lens and its real image is formed at $20 c m$ away from the lens. Calculate the focal length of convex lens. Draw ray diagram also.

Here,
Image distance $= v = 20 c m$ ,
Object distance $= u = - 30 c m$ ,
Lens equation is given by,
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$ ,
putting the values we get,
$\frac{1}{20} - \frac{1}{- 30} = \frac{1}{f}$ ,

$∴ f = 12 c m$

$∴$ Focal length of convex lens $= f = 12 c m$