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Question

I saw a question that a person is suffering from myopia and he has a far point of 80cm. In its solution it is mentioned that the object distance is infinity. How come it be? 


Solution

The far point of a normal eye is infinity. But, it shifts to a certain distance in a myopic eye. Hence, the image of an object at infinity needs to be formed at a point which is the far point of myopic eye. Hence the value of u is taken as infinity.

​​​​here is the solution,
    

As the defect is myopia so we need a diverging lens and the power of lens can be calculated as follows:

v=-80cm,u=-infinity (bcoz the person can see to the specific distance which is as at infinfinty for him)

we know that:1/f=1/v-1/u

so substituting the values:

1/f=-1/80-(-1/infinity)

1/f=-1/80+0(1/infinity=0)

1/f=-1/80

therefore,f=-80cm or -0.8m

now ,P=1/f

P=-1/0.8

P=-1.25D


 

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