Question

A fly appears to be upside down and 4 times its actual size when viewed through a converging lens with a focal length of 4 cm. How far is the fly from the lens?

Answer 1

Solution :

It is a very simple problem.

We know Linear Magnification (m) produced by lens is defined as the ratio of size of image with respect to size object.

m = I / O

m = 4 (given in Question)

We also know Linear Magnification  produced by lens is also given by m = V / U

m = Image distance / Object Distance == Height of Image / Height of Object

m = V / U = I / O

4 = V / U   V = 4 U

From Lens formula,

1 / f = 1 / V - 1 / U

Focal Length f = 4 cm (given)

so,

1 / 4 = 1 / V - 1 / U

1 / 4 = 1 / 4U - 1 / U (since V = 4U)

U = -3 cm

i.e. Object is 3 cm far from lens.Negative sign shows that object is on Left side of converging lens.

Answer : -3cm or 3cm on left side.