Question

Which of the following options is /are correct regarding the Lens- displacement method. $[D=\text{distance between object and screen},f=\text{focal length of lens}]$

• A. The method is suitable only for convex lenses.
• B. For $D=4f$, there is only one postion where a sharp image is formed
• C. For $D<4f$, there is no position for which a sharp image is formed
• D. For $D>4f$, there is no position for which a sharp image is formed

Answer 1

Answer: (A), (B), (C)

For $D<4f$, there is no position for which a sharp image is formed

As per displacment method.
$d=\sqrt{D^2-4Df}=\sqrt{D(D-4f)}$
where d = distance between two different Position of lens.
we know that, solution for $d$ is possible only when $D\ge 4f$

$\therefore$ When , $D<4f\to$ There is no Position for which sharp image can be formed.

When , $D=4f\to$ There is only one Position where a sharp image is formed.

When ,$D>4f\to$ There are two positions where a sharp image is formed.

And also, This method is suitable only for convex lenses.

Hence, options (a) , (b) and (c) are the correct answers.