Question

if the light wave has a wavelength of 10m what would be its velocity

Answer 1

If this case could ever happen, the speed would follow from this formula:

$v = f \cdot \lambda$

with f the frequency and lambda the wavelength. We are give a wavelength of 10m. The frequencies of the visible light can range between 400 to about 790 Terahertz, so let us pick a middle point of 600 THz ("green-ish") as a "representative."

$v = 600THz\cdot 10m = 6\cdot 10^{14} \frac{1}{s}\cdot 10 m = 6\cdot10^{15}\frac{m}{s}$

The speed of such a wave would have to be 6e+15 m/s (which would be 7 orders of magnitude higher than the universal speed of light constant)

Answer 2

The speed of light ... and all other forms of electromagnetic radiation ... depends only on the electrical properties of the medium, and not on wavelength.

The speed of light is

(299,792,458 m/s) / (index of refraction of the medium)

(The index of refraction of vacuum is 1.00 .  It's greater than 1.00 in any material stuff, and different in every material.)

= = = = =

By the way, an electromagnetic wave 10 meters long is no light wave.  It's a radio wave whose frequency is 30 MHz.