光譜的波長是甚麼呢？認如何正確理解波長呢？

Electromagnetic (EM) waves are characterized by their wavelengths. EM waves of different wavelength gives effects in respect to various physical phenomena such as diffraction, interference, penetration, absorption in media.... etc.
In the optical region, different wavelengths of light appear as different colours to our eyes. Waves of longer wavelength appear as infrared radiation, which give us the sensation of heat. On the other hand, waves of shorter wavelengths beyond the optical spectrum appear as ultraviolet, which cause tanning to our skin.
Very short wavelength EM waves have special characteristics, that is, ionization. These waves demonstrate themselves as X-rays or gamma rays.
Waves of other wavelengths could serve other purposes due to their different wavelength characteristics, e.g. wavelengths of radio waves are best used for radio communication or broadcasting because of prominent diffraction effect.
Therefore, wavelength of a wave gives different physical properties to it. It is this special property that the wave is being utilized.