a short sighted person can see most clearly at a distance of 15 cm,acquires spectacles enabling him to see most clearly objects at a distance of 60 cm. calculate the total length of the lens and power of the lens
focal length is given as
1/f = 1/v - 1/u
here
u = -60 cm
v = -15 cm
so,
1/f = (1/-15) - (1/-60)
1/f = (-4 + 1)/60
or focal length
f = -60/3 cm = -20 cm
and power of lens
[here f = -20cm = -0.2 m]
P = 1/f = 1/-0.2 = -5D