A monochromatic ray of light incident on one refracting surface of an equilateral prism, suffers a…

From Figure 7, the refracted ray inside the equilateral prism is parallel to the base, which means the prism is in the minimum deviation position ($\delta_m = 40^\circ$). For an equilateral prism, angle of prism $A = 60^\circ$. Using the prism formula: $\mu = \frac{\sin\left(\frac{A + \delta_m}{2}\right)}{\sin\left(\frac{A}{2}\right)}$ $\mu = \frac{\sin\left(\frac{60^\circ + 40^\circ}{2}\right)}{\sin\left(\frac{60^\circ}{2}\right)} = \frac{\sin 50^\circ}{\sin 30^\circ} = \frac{0\cdot766}{0\cdot5} = 1\cdot532 \approx 1\cdot53$

Dispersive power ($\omega$) of a transparent material is defined as the ratio of angular dispersion between two extreme colours (violet and red) to the mean deviation produced by a thin prism of that material: $\omega = \frac{\delta_V - \delta_R}{\delta_Y} = \frac{\mu_V - \mu_R}{\mu_Y - 1}$