Dictionary
Radiation - Light

All objects emit light at characteristic wavelengths, or in other words, an electro-magnetic spectrum (Click here to know more about the electro-magnetic spectrum). This light is always emitted in straight lines (click here to know more about emission) and spreads out over a larger and larger area as it travels: the light per unit area diminishes as the square of the distance. When light strikes an object with a rough surface, it is either absorbed or scattered in all directions (click here to know more about absorption and reflection). Some frequencies are absorbed more than others, and this gives objects their characteristic color.
But exactly, what is light? Two main theories have been developed: the English mathematician and physicist Sir Isaac Newton described light as an emission of particles, and the Dutch astronomer, mathematician, and physicist Christiaan Huygens developed the theory that light travels by a wave motion. It is now believed that these two theories are essentially complementary. In fact, depending on the experimental conditions, light can act like a series of particles or like a wave.

Therefore, Light can be seen in two ways:

Where is the wavelength, the distance between two crests.
The frequency ( or f ) tells how many crests pass during each time interval

Light is unique among waves, since it doesn't need a medium to propagate: it can travel in vacuum, while sound cannot. Furthermore, the speed of light is the ultimate speed limit: nothing in our Universe can travel faster.

where c is the speed of light ()
and
h is a constant called Planck's constant.

From this formula, we know that the higher the frequency of light, the shorter its wavelength and the higher its energy. X-rays or gamma-rays are examples of high frequency and high energy light. On the other hand, Radio waves are examples of light with a long wavelength, low frequency, and low energy.