1.

How can we find the maximum and minimum focal lengths of the eye lens ?

Answer»

Solution :a) When the OBJECT is at infinity.
`u=- oo, v=2.5` cm (image DISTANCE which is equal to distance between eye lens and retina ) ltbRgtUsing the formula `1/f =1/v -1/u =1/(f_("MAX")) =1/(2.5)-1/oo implies1/(f_("max")) =1/(2.5)+0`
`therefore f_("max")=2.5 cm`
b) Ojbect at a distance of 25 cm from eye. In this case eye has minimum focal LENGTH.
Here u=-25 cm, v=2.5 cm
Using the formula `1/f =1/v -1/u`
`implies f_("max")=1/(2.5) +1/(25) implies1/(f_("max")) =(10)/(25) +(1)/(25) implies1/(f_("min")) =(10+1)/(25)implies 1/(f_("min"))=(11)/(25)`
`therefore f_("min")=(25)/(11)=2.27` cm


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