THE EM SPECTRUM
Since all EM waves travel at the same rate in a vacuum, wavelengths and frequencies must be changing. The wave with the longest wavelength is the Radio Wave and the wave with the shortest wavelength and highest frequency is a Gamma Ray. The EM spectrum is best explained in a visual way (Regard the visual aid down below).
Visual AId
ACtivities
If you're still having trouble getting use to the EM Spectrum, here are some fun activities and videos to help you out (for the BrainPOP video the username is asdubai and the password is falcons).