An object is placed at a distance of $1.50 \, m$ from a screen. $A$ convex lens is placed between them to produce a four times magnified image on the screen. The focal length of the lens is ..... $cm$.

  • A
    $24$
  • B
    $12$
  • C
    $84$
  • D
    $150$

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$A$ biconvex lens has a radius of curvature of magnitude $20\, cm$. Which one of the following options best describes the image formed of an object of height $2\, cm$ placed $30\, cm$ from the lens?

The thickness at the centre of a plano-convex lens is $3 \, mm$ and the diameter is $6 \, cm$. If the speed of light in the material of the lens is $2 \times 10^{8} \, m/s$,the focal length of the lens is ............ $cm$.

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