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(b) Sunita's ophthalmologist suggests her to use a lens of power -2 D to correct her vision.
(i) What type of lens should she use?
(ii) What should be the focal length of the lens?
(iii) An object is kept at 10 cm in front of the lens of power -2 D. Find the distance where the image is produced.
Answer:
(b)
(i)
-2D power = - ½ metres focal length
Concave lenses have negative focal lengths .
Therefore, she should use a diverging lens/ concave lens .
(ii)
f = 1/P = 1/(-2) = - 0.5 m
(iii)
Given :
u = -10 cm
P=-2 D
f = -0.5m = -50 cm
1/f = 1/v - 1/u
v = uf/u+f
v = (-10)(-50)/-10 -50
v = -8.33 cm
Therefore, the image is produced 8.33cm in front of the lens .
