For two identical slits (each intensity $I_0$): $I = 4I_0\cos^2\!\left(\frac{\phi}{2}\right)$

• $I_\text{max} = 4I_0$ at $\phi = 0, 2\pi, 4\pi, \ldots$ (constructive, path diff = $0, \lambda, 2\lambda, \ldots$)
• $I_\text{min} = 0$ at $\phi = \pi, 3\pi, 5\pi, \ldots$ (destructive, path diff = $\lambda/2, 3\lambda/2, \ldots$)

Energy conservation: Average intensity $\langle I \rangle = 2I_0 = I_1 + I_2$. Interference does not create or destroy energy — it redistributes it.

Fringe visibility: $V = \frac{I_\text{max} - I_\text{min}}{I_\text{max} + I_\text{min}} = 1 \quad \text{(identical slits, perfect contrast)}$

For unequal slits ($I_1 \neq I_2$): $V < 1$; for completely blocked slit: $V = 0$.

Key intensity values: