Which of the following is not true for electromagnetic waves?

Electromagnetic wave have constant velocity in air/vacuum and it is independent of frequency.

The velocity is given by:$\mathrm{c}=\sqrt{\frac{1}{{\mu }_0{ϵ}_0}}=3\times {10}^8 \mathrm{m}/\mathrm{s}$.

In a medium, $\mathrm{v}=\sqrt{\frac{1}{{\mu }_r{ϵ}_r}}\simeq \sqrt{\frac{1}{{\mu }_0{ϵ}_0{ϵ}_r}}=\frac{c}{\sqrt{{ϵ}_r}}$

so, in a medium, speed depends on frequency/wavelength of the EM waves.