The electromagnetic spectrum can be by wavelength or by frequency. For visible light, blue light has the shortest wavelength, red light has the longest.
Red light has the lowest frequency, blue light has the highest.
Exploring spectra with a spectrometer and a diffraction grating. The line spectra of atoms are produced because electrons in atoms exist in energy levels. These energy levels have well defined energies. When the electrons transition from one energy level to another they emit a photon with a precise energy. Investigate the energy levels of an atom by finding the energy levels of a stool and a croquet mallet.