A) D/8
B) D/4
C) D/2
D) D
Correct Answer: A
Solution :
Q \[\mu =a+\frac{b}{{{\lambda }^{2}}}\] (Cauchy's equation) and dispersion \[D=-\frac{d\mu }{d\lambda }\] Þ \[D=-\,(-\,2{{\lambda }^{-3}})b=\frac{2b}{{{\lambda }^{3}}}\] Þ \[D\propto \frac{1}{{{\lambda }^{3}}}\] Þ \[\frac{D'}{D}={{\left( \frac{\lambda }{\lambda '} \right)}^{3}}=\left( \frac{\lambda }{2\lambda } \right)=\frac{1}{8}\] Þ \[D'=\frac{D}{8}\]
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