A lens has a power of +5 D in air. What will be its power if completely immersed in water? (refractive index of water w.r.t air = 4/3 and refractive index of glass w.r.t. air = 3/2)

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Answer 1 · 2018-02-15T14:51:21

A lens with radius of curvature #R_1# & #R_2# having relative refractive index of #mu#,if have a power of #D# ,then these are related as,

# D= (mu-1)((1/(R_1))-(1/(R_2)))=(mu-1)×k# (let)

Now for the lens in air,we can write,

#5 = (((3/2)/1)-1) × k#

Or, #k=10#

Now,when it is put in water,#mu= (3/2)/(4/3)=(9/8)#

So,if now the power becomes #D# then, #D=((9/8)-1)×k = 1/8 × 10=1.25 D#