Identify the given critical angle.

$$
c = 42^\circ
$$

$$
n = \frac{1}{\sin c}
$$

$$
n = \frac{1}{\sin 42^\circ}
$$
$$
n \approx \frac{1}{0.6691}
$$
$$
n \approx \boxed{1.49}
$$

**b)** Two conditions necessary for total internal