When passing through an equilateral transparent prism, the minimum...

The dispersive power $\omega$ of a prism is defined as the ratio of angular dispersion to the mean deviation.

Angular dispersion is given by the difference in deviation of the extreme colours:

$\theta = \delta_v - \delta_r$

Mean deviation is given by the deviation of the yellow light:

$\delta = \delta_y$

Therefore, the dispersive power is:

$\omega = \dfrac{\delta_v - \delta_r}{\delta_y}$

Substituting the given values:

$\omega = \dfrac{39.2^{{}^{{}^{\circ}}} - 38.4^{{}^{{}^{\circ}}}}{38.7^{{}^{{}^{\circ}}}}$

$\omega = \dfrac{0.8^{{}^{{}^{\circ}}}}{38.7^{{}^{{}^{\circ}}}}$

$\omega \approx 0.0206$