Question

A farsighted person cannot focus objects closer than 100 cm the lens that will permit him tobread from a dist of /40 cm hasva focal length of

Answer 1

Answer:

Explanation:

Given A farsighted person cannot focus objects closer than 100 cm the lens that will permit him to read from a distance of 40 cm have a focal length of

So a farsighted person uses a convex lens

So u will be - 40 cm, v = 100 cm

We know that

1/f = 1/v - 1/u

1/f = 1/100 - (-1/40)

1/f = 1/100 + 1/40

1/f = 2 + 5 / 200

1/f = 7/200

or f = 200 / 7

f =  28.57 cm

So focal length will be 28.57 cm

Answer 2

Answer:-

Given

V= 100 cm

u = 40 cm

focal length is given as

1/f = 1/v - 1/u

here

u = -40 cm

v = -100 cm

so,

1/f = (1/-100) - (1/-40)

1/f = (-2+5)/200

1/f = 3/200

or focal length

f = 200/3 cm = 66.67 cm

and power of lens

[here f = 66.67 cm = 0.66m]

P = 1/f

= > 1/0.66

=> 1.51 D

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@GauravSaxena01