Question

  A sharp image is located 391 mm behind a 215-mm focal- length converging lens. Find the object distance (a) using a ray diagram, (b) by calculation.

 

Answer 1

Solution

Image distance

$d_i=391mm$

Focal length f=215mm

the object distance

a) so diagram can shown as

So object distance is $d_0=480mm.$

b) now using the lens formula

$\frac{1}{d_0}+\frac{1}{d_i}=\frac{1}{f}$

Puting all values

$\frac{1}{d_0}+\frac{1}{391}=\frac{1}{215}$

$\frac{1}{d_0}=-\frac{1}{391}+\frac{1}{215}$

After solve object distance

$d_0=480mm.$