Of all the colors in the visible spectrum, red light is the least energetic and blue is the most. Beyond the red end of the visible part of the spectrum lie infrared and radio light, both of which have lower energy than visible light. Because of light's unique properties, it can be described not only in terms of its energy, but also its wavelength, or its frequency. This is a scientific convention that allows the use of the units that are the most convenient for describing whatever energy of light you are looking at. For example, it would be inconvenient to describe both low energy radio waves and high energy gamma-rays with the same units because the difference in their energies is so great.
Frequency is the number of cycles of a wave to pass some point in a second.
Since light can act like both a particle and a wave, we say that light has particle-wave duality. We call particles of light photons . This is an important difference because it affects the way we build instruments to measure light.