(A) Using the lens formula: $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$.Given: Object distance $u = -25 \ cm$ (by sign convention) and image distance $v

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(A) Using the lens formula: $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$.Given: Object distance $u = -25 \ cm$ (by sign convention) and image distance $v = +75 \ cm$ (since the image is formed on a screen on the other side).Substituting the values:$\frac{1}{f} = \frac{1}{75} - \frac{1}{-25} = \frac{1}{75} + \frac{1}{25}$.$\frac{1}{f} = \frac{1 + 3}{75} = \frac{4}{75}$.$f = \frac{75}{4} = +18.75 \ cm$.Since the focal length is positive,the lens is a convex lens.

Class 12 Physics Ray Optics and Optical Instruments Refraction by Lenses

Medium

$A$ candle placed $25 \ cm$ from a lens forms an image on a screen placed $75 \ cm$ on the other side of the lens. The focal length and type of the lens should be

KCET 2018 All KCET

A
$+18.75 \ cm$ and convex lens
B
$-18.75 \ cm$ and concave lens
C
$+20.25 \ cm$ and convex lens
D
$-20.25 \ cm$ and concave lens

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Ray Optics and Optical Instruments

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Refraction by Lenses

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$A$ candle placed $25 \ cm$ from a lens forms an image on a screen placed $75 \ cm$ on the other side of the lens. The focal length and type of the lens should be

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