The eye can be regarded as a single refracting surface. The radius of curvature of this surfac is equal to that of cornea (7.8 mm). This surface separates two media of refractive indices 1 an 1.34. Calculate the distance from the refracting surface at which a parallel beam of light wil come to focus.

Options

A. 4.0 cm
B. 1 cm
C. 3.1 cm
D. 2 cm

Answer

A, C

Explanation

`n_1=1.0`
`n_2=1.34`
`R_1=7.8 mm=0.78`
`R_2=infty`
For parallel beam,
`u~~infty`
v=f (Image is obtained at focus)
`-1/u+1/v=(n_2-n_1)/(n_1)((1)/(R_1)-(1)/(R_2))`
`therefore (-1)/(infty)+1/f=(1.34-1)/(1.34)((1)/(0.78)-(1)/(infty))`
`therefore1/f=(0.34)/(1.34)((1)/(0.78))`
`therefore f=3.07=3.1` cm