Today We will prove that light acts like a wave, again. This time we will shine our laser on 2 closely spaced slits. E Let's assume that they are very narrow, so that the single slit interference pattern is not noticeable. Huygen's Principle The narrow double slits act like 2 distinct point sources of waves. We will again get an interference pattern Lets look more carefully. How does the interference pattern emerge? Constructive Interference Destructive Interference Dark and bright spots alternate The Math. Light from S 2 has to travel farther than light from S 1 . The path difference is ?=d sin?. Constructive interference occurs at point P if ? =0, or if ? is a multiple of the wavelength ?. Æ d sin? = m?, m=0, ±1, ±2, ±3,. Double Slit Interference . The rings cross at some places, giving extra amplitude to the wave . A pattern of bright and dark spots emerge Double Slit Interference . Amazingly, light is not the only thing that acts as both a ...