Which property of light waves is responsible for what we see as brightness or intensity?

Electromagnetic radiation is one of many ways as energy travels through space.
 As you may already know, a wave has a valley (lowest point) and a ridge (highest point). The vertical distance between the tip of the crest and the central axis of the wave is known as amplitude. This is the property associated with the brightness, or intensity, of the wave. The horizontal distance between two consecutive crests or valleys of the wave is known as wavelength