A long-sighted person cannot see objects nearer to his eye than 50 cm. To enable him to read a book 25 cm away, he should use spectacle lenses whose power in dioptres is?

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Solution

For focal length u=(-25cm)

v=(-50cm)

(lens formula)

1/f=1/v -1/u

=(-1/50 - (-1/25)

=-1/50 + 1/25

1/f=1/50

f=+50cm

p=1/f

=100/50

p=+2 D

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A long-sighted person cannot see objects nearer to his eye than 50 cm. To enable him to read a book 25 cm away, he should use spectacle lenses whose power in dioptres is?

For focal length u=(-25cm) v=(-50cm) (lens formula) 1/f=1/v -1/u =(-1/50 - (-1/25) =-1/50 + 1/25 1/f=1/50 f=+50cm p=1/f =100/50 p=+2 D

For focal length u=(-25cm)

v=(-50cm)

(lens formula)

1/f=1/v -1/u

=(-1/50 - (-1/25)

=-1/50 + 1/25

1/f=1/50

f=+50cm

p=1/f

=100/50

p=+2 D