Question

The image of an illuminated square is obtained on a screen with the help of a converging lens . The distance of the square from the lens is 40 cm . The area of the image is 9 times that of square . The focal length of lens is ?

Answer 1

I am getting two possible answers : 30 cm and 60 cm , correct one is 30 cm.

Linear magnification is either +3 or -3.
v/u=+3 or v/u=-3.
v=-120cm or v=120cm
Using lens formula :1/f=1/v-1/u
f=60cm or f=30cm.

The answer is 30cm , What am i missing ?

Answer 2

@Mashy , could you please help me out ?

Answer 3

There is only one case!
linear magnification cannot be +3 or -3
it has to be one of them.. which one? :P

Clue:
The image is obtained on the screen :P

Answer 4

So its an real image. Right?

Answer 5

Becsuse we can c the real image on the screen .

Answer 6

So for real image
m=+3?
Or
m=-3?

Answer 7

Hahah! Thanks @Mashy ! silly me !
m=-3 :)