Assertion: Different colours travel with different speed in vacuum.
Reason : Wavelength of light depends on the refractive index of the medium.
A) Both assertion and reason are true but the reason is the correct explanation of assertion
B) Both assertion and reason are true but the reason is not the correct explanation of assertion
C) Assertion is true but reason is false
D) Both the assertion and reason are false
E) Reason is true but assertion is false

Hint:As we know that the speed of light is roughly in the order of $3 \times {10^8}\;m/s$ and it is greatly affected by the medium in which it is traveling. The speed of light can scatter from the matter of the medium it is traveling.

Complete step by step answer:
We know that all the colours in the spectrum that is VIBGYOR travels with the same speed in the vacuum as there is no medium or air.
Therefore, the assertion is false.
But we know that the wavelength of light $\lambda $ varies inversely with the refractive index $\mu $ as:
$\lambda \propto \dfrac{1}{\mu }$
Therefore, the reason is true.

Therefore, the correct option is E.

Additional Information: We should keep in mind that the speed of light changes while it travels in a different medium. When a light wave propagates through a medium it is obstructed by the atoms of the medium. When the light hits the atom of the medium it is absorbed firstly and then afterward it is re-emitted and this process goes on again and again which takes a certain amount of time to travel. The more the light is obstructed, the slower is the effective average speed.

Note: We must know that the medium in which speed of light is less is the optically denser medium whereas the optically rarer medium is one where the speed of light is maximum. The light scatters less in the rarer medium and that’s why it’s speed is more in the rarer medium.