where d is the distance to the detection screen, a is the separation between the two slits, x the distance to the center of the screen, λ the wavelength and dN / dt is the number of photons emitted per unit time at the source. The amplitude of measuring a photon at distance x from the center of the screen is the sum of these two amplitudes coming out of each hole, and therefore the probability that a photon is detected at position x will be given by the square of this sum: I ( x ) = | f 1 ( x ) + f 2 ( x ) | 2 = | f 1 ( x ) | 2 + | f 2 ( x ) | 2 + [ f 1 ∗ ∗ ( x ) f 2 ( x ) + f 1 ( x ) f 2 ∗ ∗ ( x ) ] ,