What does it mean for electromagnetic waves to travel at the speed of light?

Electromagnetic waves, including visible light, travel at a constant speed in a vacuum, which is approximately 299,792 kilometers per second (or about 300,000 kilometers per second, often rounded for simplicity). This speed is commonly referred to as the speed of light. In vacuum, all types of electromagnetic radiation— radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays—travel at this same constant speed.

When electromagnetic waves enter different media, such as water or glass, their speed changes due to interactions with the atoms and molecules in that medium. This is why light travels slower in water or glass compared to its speed in a vacuum. Thus, it’s incorrect to assert that these waves have a constant speed regardless of the medium. Similarly, when light transitions from one medium to another, such as from air to a gas, it does not necessarily slow down in every scenario; the speed changes based on the medium's characteristics.

Understanding the nature of electromagnetic waves’ speed highlights the distinction between their behavior in a vacuum versus various media, reinforcing why they are fastest in a vacuum and why their speed can be affected by environmental factors.