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Question
1. The near point of a hypermetropic person is 75 cm from the eye. What is d power of d lens required to enable him to read clearly a book held at 25 cm from the eye?
2. If the person in the previous problem uses spectacles of power +1.0 D wat is d nearest dist. of distinct vision for him?
1. The near point of a hypermetropic person is 75 cm from the eye. What is d power of d lens required to enable him to read clearly a book held at 25 cm from the eye?
2. If the person in the previous problem uses spectacles of power +1.0 D wat is d nearest dist. of distinct vision for him?
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Solution
1. Defective near vision, v = -75cm
Now, lens formula for this,
1/v + 1/25 = 1/f
=> 1/(-75) + 1/25 = 1/f
=> 2/75 = 1/f
=> f = 75/2 cm
So, power of the lens, P = 1/f
=> P = (100/(75/2)) D
=> P = 2.667 D
2. Given P = 1D
f = 1m = 100cm
Use formula,
1/v + 1/25 = 1/f
to find v = .........?
1. Defective near vision, v = -75cm
Now, lens formula for this,
1/v + 1/25 = 1/f
=> 1/(-75) + 1/25 = 1/f
=> 2/75 = 1/f
=> f = 75/2 cm
So, power of the lens, P = 1/f
=> P = (100/(75/2)) D
=> P = 2.667 D
2. Given P = 1D
f = 1m = 100cm
Use formula,
1/v + 1/25 = 1/f
to find v = .........?
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