The speed of light in a vacuum is exactly 299,792,458 meters per second. Physicists write it as c. For quick estimates, 300,000 kilometers per second is a convenient rounded value. The exact number is built into the definition of the meter. When converting units, divide the meters per second by one thousand to get kilometers per second. Keep the distinction between the exact value and the rounded one if you are reporting a precise calculation rather than estimating a travel time.

To estimate light travel time, divide the distance by the speed, using matching units. Take a rounded Sun–Earth distance of 150 million kilometers and divide by 300,000 kilometers per second. The result is about 500 seconds, or 8 minutes and 20 seconds. Earth’s distance from the Sun varies over the year, so this is a useful approximation rather than a constant arrival time. The calculation gives a one-way trip. A signal sent out and reflected back would have to cover both legs.

Light propagates more slowly through ordinary transparent materials such as water and glass than it does in a vacuum. When it enters water at an angle, the change in speed can bend its path. This is refraction. Try looking at a pencil in a clear glass partly filled with water. From the side, the underwater part can appear shifted because rays leaving the water change direction at the boundary. Changing your viewing angle changes the apparent shift. The pencil stays straight throughout the demonstration.