The variation in brightness of the reflected light as a function of gap width formula_1 can be found by deriving the formula for the sum of the two reflected waves. Assume that the "z"-axis is oriented in the direction of the reflected rays. Assume for simplicity that the intensity "A" of the two reflected light rays is the same (this is almost never true, but the result of differences in intensity is just a smaller contrast between light and dark fringes). The equation for the electric field of the sinusoidal light ray reflected from the top surface traveling along the z-axis is