(a) Compare the speeds of light of wavelength 4000 Angstrom  (i.e. violet light) and 8000 Angstrom (i.e. red light) in vacuum.

Solution:

(a) Because the speed of light is determined by the medium rather than the wavelength, the speed will be the same for both.

(b) Because a denser medium has a greater refractive index, the speed of light in that medium is slower than the speed of light in a medium with a lower refractive index.