Question

Which colour of light has the highest speed?

Answer 1

1. Light is an electromagnetic (EM) wave, and EM waves of all wavelengths (colours) have a constant speed of $c=3\times {10}^8\mathrm{m}/\mathrm{s}$ in vacuum.
2. Light through a medium have slightly lower speed and it is dependent on the refractive index of the medium.
3. The expression for the speed is $v=\frac{c}{n}$, where, $v$ is the speed in the medium, $c$ is the speed in vacuum and $n$ is the refractive index of the medium.
4. The refractive index of the medium in turn in dependent on the wavelength of light as given by the expression, $n=A+\frac{B}{{\lambda }^2}$. Here, $A$ and $B$ are constants and $\lambda$ is the wavelength of light.
5. A larger value of the wavelength $\lambda$ will result in a lower value of the refractive index $n$ and in turn gives a higher value of the speed $v$.

Hence, the speed of light in vacuum is same for all colors but red coloured light has the highest speed in a dielectric medium as it has the largest wavelength.