The speed of light is used as the velocity when you are calculating the wavelength of electromagnetic waves traveling in a vacuum or air (where air's refractive index is close to 1). This is because electromagnetic waves, such as visible light, radio waves, or X-rays, travel at the speed of light in these mediums.

The relationship between the speed of light ($c$), wavelength ($\lambda$), and frequency ($f$) of the wave is given by the equation:

$c = \lambda \cdot f$

Where:

• $c$ is the speed of light ($3 \times 10^8 \, \text{m/s}$ in vacuum),
• $\lambda$ is the wavelength,
• $f$ is the frequency.

When to use the speed of light in vacuum:

1. When calculating the wavelength of light or other electromagnetic waves in vacuum.
2. For approximations in air, since the speed of light in air is very close to that in vacuum.
3. When working with light or electromagnetic waves in outer space or interstellar mediums (unless otherwise specified).
4. If you are calculating wavelength for any wave that is assumed to propagate without significant medium interaction (ideal conditions).

However, if the wave is traveling through another medium, such as water, glass, or any other material, the speed of the wave changes due to the medium's refractive index, and you must account for this by using the velocity in that particular medium.

Do you need more details on this topic, or have any other questions?

Related questions:

1. How does the wavelength change when light enters a different medium?
2. What is the refractive index, and how does it affect wave velocity?
3. How can we calculate the frequency of a wave given its wavelength?
4. Why is the speed of light constant in a vacuum?
5. What is the difference between phase velocity and group velocity in waves?

Tip: Always check the medium before assuming the speed of light as $3 \times 10^8 \, \text{m/s}$ in your calculations.