What is the speed of light in air? In crown glass?

Light travels 1.0003 times slower in the air than it does in a vacuum, slowing it from 299,792,458 meters per second to 299,702,547 meters per second. That's a slowness of 89,911 meters per second, which may appear significant but is merely three-tenths of the speed of light.

The relationship $n=\frac{c}{v}$modifies the speed of light in materials by altering its coefficient of refraction. The coefficient of refraction of air is $n_a=1.000293$, while that of glycerin is $n_g=1.52$, according to the values in the table 25.I. Accordingly,

$$\begin{gathered} v_a=\frac{c}{n_a} \\ =\frac{2.997925\times {10}^{8}m/s}{1.000293} \\ =2.999047\times {10}^{8}m/s \\ {v}_g=\frac{c}{n_g} \\ =\frac{3.00\times {10}^{8}m/s}{1.52} \\ =1.97\times {10}^{8}m/s \end{gathered}$$

In the case of the air, more significant values are required to demonstrate the minute change. The speed of light in air is $v_w=3.00\times {10}^{8}m/s$. The speed of the light in the crown glass is $v_g=1.97\times {10}^{8}m/s$.