Question

If the refractive indices of crown glass for red, yellow and violet colours are respectively $\mu_{R}, \mu_{Y}$ and $\mu_{V},$ then the dispersive power of this glass would be

• A. $\frac{\mu_{V}-\mu_{Y}}{\mu_{R}-1}$
• B. $\frac{\mu_{V}-\mu_{R}}{\mu_{Y}-1}$
• C. $\frac{\mu_{V}-\mu_{Y}}{\mu_{Y}-\mu_{R}}$
• D. $\frac{\mu_{V}-\mu_{R}}{\mu_{Y}}-1$

Answer 1

Answer: (B)

$\omega=\frac{\delta_{V}-\delta_{R}}{\delta_{\text {mean }}}$
$=\frac{\left(\mu_{V}-1\right) A-\left(\mu_{R}-1\right) A}{\left(\mu_{Y}-1\right) A}$
For $\delta_{\text {mean }},$ we take yellow as mean of violet and red.
$\Rightarrow \omega=\frac{\mu_{V}-\mu_{R}}{\mu_{Y}-1}$