An old age person can see object placed between 75 cm and 5m. What is the power of the corrective lens required???
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A person suffering from hypermetropia can see distinct objects clearly but faces difficulty in seeing nearby objects clearly. It happens because the eye lens focuses the incoming divergent rays beyond the retina. This defect of vision is corrected by using a convex lens. A convex lens of suitable power converges the incoming light.
The given person will be able to clearly see the object kept at 25 cm (near point of the normal eye), if the image of the object is formed at his near point, which is given as 1 m.
Object distance, u = -25 cm
Image distance, v = -50cm
Focal length, f
Using the lens formula,
1/f = 1/v - 1/u
here u = -25 cm
v = -50cm
substituting & calculating for f, we get
f = +50cm = +0.5 m
Power P = 1/f = +2 D
Hope that resolves your problem.
Best Wishes
The given person will be able to clearly see the object kept at 25 cm (near point of the normal eye), if the image of the object is formed at his near point, which is given as 1 m.
Object distance, u = -25 cm
Image distance, v = -50cm
Focal length, f
Using the lens formula,
1/f = 1/v - 1/u
here u = -25 cm
v = -50cm
substituting & calculating for f, we get
f = +50cm = +0.5 m
Power P = 1/f = +2 D
Hope that resolves your problem.
Best Wishes
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