consider light getting refracted by a prism, the red light with the least frequency gets refracted the least. and the violet with the highest frequency gets refracted the most. now consider skywave propagation. the waves with higher frequencies escape without being refracted and lost into atmosphere. and those with lower frequencies get refracted enough to return to earth which contradicts what we saw in prism. The reason i cud thin of was that the refractive index of ionosphere maybe less than 1 or even negative and the principles of refraction gets reversed or somethng... but this is not a solid thought. can you help me ?