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Solution


Q.

What is the relative decrease in focal length of a lens for an increase in optical power by 0.1 D from 2.5 D ['D' stands for dioptre]          [2025]
1 0.04  
2 0.40  
3 0.1  
4 0.01  

Ans.

(1)

When P = 2.5 D

F=1P=12.5

When P'=2.6 D

F'=1P'=12.6

Relative decrease in focal length

F–F'F=25–51325=1–2526=126=0.04