The frequency of the waves increases as you go from left to right on the El. spectrum / The length of the waves decreases as you go from left to the right on the El. spectrum - both of them are correct use the one you prefer more
P.S. most textbooks should show that radiowaves are on the left side and UV or gamma rays on the right - if it's opposite in your one then just replace the second word with their appropriate antonyms.