Solve this:

26. For achromatic combination of lenses, if we use two lenses to focal length length and respectively and dispersive power  ω  and 
ω '. then.
( 1 )   ω = ω ,           ω ' = 2 ω ,   f ' = 2 f           ( 2 )   ω = ω ,           ω ' = 2 ω ,   f ' - 2 f ( 3 )   ω = ω ,           ω ' = 2 ω ,   f ' = f 2           ( 4 )   ω = ω ,           ω ' = 2 ω ,   f ' = - f 2

Dear Student ,
As we know that in case of the achromatic combination of lenses product of the sum of their power and the dispersive is equal to zero .
Therefore we can write that ,ωf+ω'f'=0Now the above will only satisfy if we take that ,ω=ω0 , ω'=2ω0 and f'=-2fThus here the option (2) is correct .Regards

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