The near point and far point of a person are 40cm and 250cm, respectively. Determine the power of lens he/she should use while reading a bool kept at
The near point and far point of a person are 40cm and 250cm, respectively. Determine the power of lens he/she should use while reading a bool kept at distance 25cm away from it,
A. 2.5D
B. 5D
C. 1.5D
D. 3.5D
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Correct Answer - C
(c ) If the object is placed at a distance 25cm from the corrected lenes then it should produce the virtual image at 40cm,
Thus, u=-25cm and v=-40cm
The lens formula is
`(1)/(f)=(1)/(v)-(1)/(u) Rightarrow (1)/(f)=(-1)/(40)+(1)/(25)`
`Rightarrow f=(200)/(3)cm=(2)/(3)m`
`therefore "Power of lens,"P=(1)/(f)=(1)/(2//3)=(3)/(2)=+1.5D`
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