A) - 20 cm
B) + 20 cm
C) -10 cm
D) + 20 cm
Correct Answer: B
Solution :
In achromatic combination, the convex lens is of crown glass and concave lens is of flint glass. Given: Dispersive power of crown glass, \[{{\omega }_{1}}=0.02\] Dispersive power of flint glass,\[{{\omega }_{2}}=0.04\] The required condition for a chromatic combination is \[\frac{{{\omega }_{1}}}{{{f}_{1}}}+\frac{{{\omega }_{2}}}{{{f}_{2}}}=0\] So \[{{f}_{1}}=-\frac{{{\omega }_{1}}}{{{\omega }_{2}}}\times {{f}_{2}}\] \[{{f}_{1}}=-\frac{0.02}{0.04}\times (-40)=20\,cm\]
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