Question

The near point of a hypermetropic eye is 45cm. Calculate the focal length and power of the lens

Answer 1

We know that , the formula is ,

1/f = 1/v - 1/u,

Normal person's Near point is 25cm,
Image distance = -25cm(- indicates direction),

=> Object distance = u = -40cm (Direction),
Focal length of Convex lens is always positive !,

Now Inputting all the values to find focal length in cm,

=> 1/f = -(1/25) -(-1/45),
=> 1/f = 1/45 - 1/25,
=> 1/f = (5-9)/200
=> 1/f = -4/200 cm,
=> f = -200/4cm, Converting it into m,
=> f = -200/4/100 = -2/4m,
focal length = -2/4 m,

Power = 1/Focal length in m ,
Units = D , Dioptre,

=> Power = 1/-2/4 = -3/4D,

Therefore power = -3/4D, and focal length = -2/4 m,

Answer 2

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