In: Physics
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?
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.