$*$ For a certain optical medium the speed of light varies from a low value of $1\mathrm{.}90{10}^8\frac{m}{s}$ for violet light to a high value of $2\mathrm{.}00{10}^8\frac{m}{s}$ for red light. $($a$)$ Calculate the range of the index of refraction of the material for visible light. $($b$)$ A white light is incident on the medium from air, making an angle of $30\mathrm{.}0$ with the normal. Compare the angles of refraction for violet light and red light. $($c$)$ Repeat the previous part when the incident angle is $60\mathrm{.}0.$ Example $23-3$