No, electromagnetic waves do not all have the same velocity. While all electromagnetic waves travel at the speed of light in a vacuum, which is approximately 299,792,458 meters per second (m/s), their speed can change when they pass through different mediums.

Here's why:

* Speed of light in a vacuum: The speed of light in a vacuum is a fundamental constant, denoted by 'c'. This is the fastest speed anything can travel in the universe.

* Speed of light in a medium: When electromagnetic waves travel through a medium like air, water, or glass, they interact with the atoms and molecules of that medium. This interaction slows the waves down. The speed of light in a medium is always less than 'c'.

* Index of refraction: The extent to which a medium slows down light is described by its refractive index. A higher refractive index means the light travels slower.

Examples:

* Light travels slower in water than in air. This is why objects appear distorted when viewed underwater.

* Light travels even slower in glass than in water, which is why lenses in glasses and cameras are made of glass.

Therefore, while all electromagnetic waves travel at the speed of light in a vacuum, their speed can vary depending on the medium they are passing through.