Complete the table for the values of sin i and sin r, draw the graph of sin i against sin r, and determine the slope.

This ratio represents the refractive index ($n$).

• For $i=20^\circ$: $\frac{0.342}{0.219} \approx 1.562$
• For $i=30^\circ$: $\frac{0.500}{0.324} \approx 1.543$
• For $i=40^\circ$: $\frac{0.643}{0.423} \approx 1.520$
• For $i=50^\circ$: $\frac{0.766}{0.500} = 1.532$
• For $i=60^\circ$: $\frac{0.866}{0.559} \approx 1.549$
• For $i=70^\circ$: $\frac{0.940}{0.602} \approx 1.561$

The slope of the graph is approximately $\boxed{1.54}$.
The slope represents the refractive index of the glass.

IV) Use the results in b (III) to calculate the velocity of light in glass, given that velocity of light in vacuum is $3.0 \times 10^8$ m/s.

n = \frac{\text{velocity of light in vacuum (c)}}{\text{velocity of light in glass (v_g)}}.

Rounding to three significant figures:
The velocity of light in glass is $\boxed{1.94 \times 10^8 \text{ m/s}}$.