A nearsighted person cannot see beyond 25 cm. Assuming that the separation of the glass from the eye is 1 cm, find the power of lens needed to see distant objects.
Answers
Given:
As the person wears spectacles at a distance of 1 cm from the eyes.
Distance of the image from the glass,
v = Distance of the image from the eye − Separation between the glass and the eye
v = 25 − 1 = 24 cm = 0.24 m
For the near-sighted person, u = ∞.
The lens formula is given by
1/v-1/u=1/f
On substituting the values in the above formula, we get:
1/f=1/-0.24-1/∞
= -42
Power of the lens, P = 1/f= − 4.2 D
Thus, the power of the lens required to see distant objects is − 4.2 D.
Given :
u = ∞ [ The lens is for near sighted which means that the object distance is more ]
v = 25 - 1
[ 1 cm apart ]
⇒ v = 24 cm
1 / f = 1 / v - 1 / u
⇒ 1 / f = 1 /- 24 - 1 / ∞
⇒ 1 / f = 1 /- 24 - 0
⇒ 1 / f = 1 / - 24
⇒ f = - 24 cm
⇒ f = -0.24 m
P = 1 / f
⇒ P = 1 / f
⇒ P = 1 / ( -0.24 )
⇒ P = -4.2 D
ANSWER:
The power is -4.2 D