The speed of light in vacuum is `3 xx 10^8 ms^(-1)`

Answer

### Step-by-Step Solution:

1. **Understanding the Speed of Light**:
The speed of light in a vacuum is a fundamental constant in physics. It is denoted as \( c \) and is approximately equal to \( 3 \times 10^8 \) meters per second (m/s). This means that light travels 300 million meters in one second when it is in a vacuum.

2. **Refractive Index**:
The speed of light can change when it travels through different mediums (like air, water, glass, etc.). This change in speed is due to the refractive index of the medium. The refractive index (n) is defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v):
\[ n = \frac{c}{v} \]
This means that if the refractive index is greater than 1, the speed of light in that medium will be less than \( 3 \times 10^8 \) m/s.

3. **Conclusion**:
In summary, while the speed of light in vacuum is \( 3 \times 10^8 \) m/s, it varies in other mediums due to their refractive indices. The refractive index indicates how much the light slows down in that medium compared to its speed in vacuum.