I was having a little trouble understanding a concept in my book. My book asked the following question: The frequency of electromagnetic wave A is twice that of electromagnetic wave B. For these two waves, which is the ratio wavelengthA/wavelengthB of the wavelengths in a a vacuum? The answer turned out to be that the ratio would equal 1/2 because both waves have the same speed. I was able to calculate the 1/2 in my ratio, but I didn't understand how both waves would have the same velocity. I know frequency * wavelenth = velocity. If frequency is twice as large in one wave, how can they have the same speed?