The speed of light in glass is slower than its speed in vacuum. The refractive index of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in the medium. The refractive index of glass is typically around 1.5, which means that light travels about 1.5 times slower in glass than it does in vacuum.

To calculate the speed of light in glass, we can use the following formula:

```

v = c / n

```

where:

* v is the speed of light in the medium

* c is the speed of light in vacuum (approximately 299,792,458 meters per second)

* n is the refractive index of the medium

For example, if the refractive index of glass is 1.5, then the speed of light in glass is:

```

v = c / n = 299,792,458 m/s / 1.5 = 199,861,639 m/s

```

Therefore, the speed of light in glass is approximately 199,861,639 meters per second.