For a normal eye, the far point is at infinity and the near point ofdistinct vision is about 25cm in front of the eye. The cornea of theeye provides a converging power of about 40 dioptres, and the leastconverging power of the eye-lens behind the cornea is about 20 dioptres. From this rough data estimate the range of accommodation(i.e., the range of converging power of the eye-lens) of a normal eye.
Given: The least distance of distinct vision is 25 cm , far point of a normal eye is infinite, the converging power of the cornea is 40 D and the least converging power of the eye-lens is 20 D .
Power of the eye-lens is given as,
P = P c + P e
Where, the power of the eye-lens is P , the converging power of the cornea is P c and the least converging power of the eye-lens is P e .
By substituting the given values in the above expression, we get
P = 40 + 20 = 60 D
Power of the eye-lens is given as,
To focus an object at the near point, object distance ( u ) is − d = − 25 cm .
f e = D = v
Where, the focal length of the eye-lens is f e ( 100 60 cm ) , the distance between the corona and the retina is D and the image distance is v .
By substituting the given values in the above expression, we get
v = 100 60 = 5 3 cm
The lens formula is given as,
1 f ′ = 1 ν − 1 u
Where f ′ is the focal length of the lens.
By substituting the given values in the above expression, we get
1 f ′ = 3 5 + 1 25 = 15 + 1 25 = 16 25 cm − 1
Power is denoted as P ' .
P ′ = 1 f ′ × 100
By substituting the given values in the above expression, we get
P ′ = 16 25 × 100 = 64 D
The power of the lens is given as,
P L = P ′ − P
By substituting the given values in the above expression, we get
Where, the power of the lens is P L .
P L = 64 - 40 = 24 D
Thus, the range of accommodation of the eye-lens is from 20 D to 24 D .
Given: The least distance of distinct vision is
Power of the eye-lens is given as,
Where, the power of the eye-lens is
By substituting the given values in the above expression, we get
Power of the eye-lens is given as,
To focus an object at the near point, object distance
Where, the focal length of the eye-lens is
By substituting the given values in the above expression, we get
The lens formula is given as,
Where
By substituting the given values in the above expression, we get
Power is denoted as
By substituting the given values in the above expression, we get
The power of the lens is given as,
By substituting the given values in the above expression, we get
Where, the power of the lens is
Thus, the range of accommodation of the eye-lens is from
