Question 275: Ray Optics

Question

The distance between an object and a screen is 100 cm. A lens can produce real image of the
object on the screen for two different positions between the screen and the object. The distance
between these two positions is 40 cm. If the power of the lens is close to $$\left( {{N \over {100}}} \right)D$$ where N is an
integer, the value of N is _________.

Accepted Answer

Using displacement method
$$f = {{{D^2} - {d^2}} \over {4D}}$$
Here, D = 100 cm
and d = 40 cm
$$f = {{{{100}^2} - {{40}^2}} \over {4(100)}} = 21\,cm$$
$$P = {1 \over f} = {{100} \over {21}}D$$
$${N \over {100}} = {{100} \over {21}}$$
$$N = 476$$

Ray Optics

Integer / Numeric Answer Type