What property of a wave determines the brightness of light?

The brightness of light is primarily determined by the amplitude of the wave. In the context of light waves, brightness is associated with the intensity of light that is perceived; higher amplitude means a greater amount of light energy is present. When the amplitude of a light wave increases, more photons are emitted per unit area, resulting in a brighter appearance to the human eye.

While frequency and wavelength are related to the color of light, they do not directly influence brightness. Frequency dictates the energy associated with the light wave, while wavelength is simply the distance between successive peaks of the wave. Phase affects the timing of the wave oscillations but does not play a role in brightness perception. Hence, amplitude is the critical factor for understanding brightness in terms of light waves.