Light is always considered to be a sinusoidal or harmonic wave without giving any reason. Maxwell's equation says that light is an EM wave but does not say its a harmonic wave or even a periodic wave. People say that every periodic or non-periodic function can be broken down as a sum of series of sine waves by using fourier analysis. But any function can be broken down as sum of series of other function. For example, any function can be broken down into a sum of series of polynomial functions using taylor series. Sine wave can be broken down as a sum of polynomial functions using taylor series. A simple sine wave can also be broken down into a sum of series of other periodic waves with various frequencies and amplitudes. So why other periodic waves such as triangle, saw tooth, square waves or other periodic waves of arbitary shapes are not considered while talking about light. While finding intensity in Young double slit experiment, the light is considered as a sine wave. If light is considered as pure triangle wave or other periodic wave, the formula of intensity as a function of distance on the screen will be very different.