Question

In lens displacement method, for the two positions of lens, lateral size of the images are $4\text{cm}$ and $9\text{cm}$ respectively. The size of the object is

• A. $3\text{cm}$
• B. $6\text{cm}$
• C. $2\text{cm}$
• D. $12\text{cm}$

Answer 1

The correct option is B $6\text{cm}$

If $l_1$ and $l_2$ are the heigths of the images in the two positions of the lens, between the object and the screen, then the length, $l$ of the object is given by,
$$l=\sqrt{l_1{l}_2}$$
Here,
$l_1=4\text{cm}$ , $l_2=9\text{cm}$

Substituting the values in the above formula,

$l=\sqrt{4\times 9}\Rightarrow l=6\text{cm}$

Hence, size of the object is, $l=6\text{cm}$

Thus, option (b) is the correct answer.