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Vision Defects and Eye Optics
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A person cannot see object clearly that are closed that $2m$ and father than $4m$ . To correct the eye vision the person will use :
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A
Bifocal lenses of power $0.5D$ and $0.25D$
💡 Explanation:
Given,
The distance of the far point $(D) = 2m$
Let f be the focal length of the eye lens.
Image distance ( distance between eye lens and retina ) for our human eye is always constant and is about
$0.25m.$
This is due to the action of ciliary muscles.
applying lens formula, we get
$f = -D = -2 m.$
But, the power of a lens is reciprocal of focal length.
Hence. Power $=\dfrac{1}{f}.$ ( f in meters )
$\dfrac{1}{-2}=0.5$D
Similarly,
The distance of the far point $(D) = 4m$
Let f be the focal length of the eye lens.
Image distance ( distance between eye lens and retina ) for our human eye is always constant and is about
$0.25m.$
This is due to the action of ciliary muscles.
applying lens formula, we get
$f = -D = -4 m.$
But, the power of a lens is reciprocal of focal length.
Hence. Power $=\dfrac{1}{f}.$ ( f in meters )
$\dfrac{1}{-4}=0.25$D
So he has to use by focal lenses