Consider a double slit experiment with photons. The two waves exiting each slit can be written as: f 1 ( x ) = d N / d t 2 π π / d 1 d 2 + ( x + a / 2 ) 2 exp ⁡ ⁡ [ i ( h / λ λ ) d 2 + ( x + a / 2 ) 2 ] , and f 2 ( x ) = d N / d t 2 π π / d 1 d 2 + ( x − − a / 2 ) 2 exp ⁡ ⁡ [ i ( h / λ λ ) d 2 + ( x − − a / 2 ) 2 ] ,