Light is the thing we define as the visible part of the spectrum of electromagnetic waves.
With the help of the impulse of a particle and the impulse of a(n) (electromagnetic) wave, evidence for light being a stream of particles and vice versa can be found.
If we take this down to a rather low level, we can say the following. Impulse = Impulse. The impulse of a "normal particle" is defined as: p = m * v.
A scientist named Louis de Broglie found that the wavelength of an electromagnetic wave can be approached with planck's constant and the impulse of the waves. The formula looks like this:
λ = h / p
p = m*v
λ = h / (m*v)
p(electromagnetic wave) = h / λ
Coming back to the first statement, Impulse = Impulse, we can conclude that h / λ = m * v or p(electromagnetic wave) = p(particle) or electromagnetic wave = particle.