Question

A real image is four times as far from a lens as is theobject.

What is the object distance, measured in focal lengths? ____from the lens

Answer 1

1.25 focal lengths

Step-by-step explanation:

The lens equation states that:

`(1)/(f)=(1)/(p)+(1)/(q)`

where

f is the focal length

p is the object distance

q is the image distance

In this problem, the image is 4 times as far from the lens as is the object: this means that

`q=4p`

If we substitute this into the lens equation and we rearrange it, we get

`(1)/(f)=(1)/(p)+(1)/(4p)=(4+1)/(4p)=(5)/(4p)\\p=(5)/(4)f=1.25 f`

so, the object distance measured in focal lengths is

1.25 focal lenghts