Lesson Plan

Explain why interference is the identifying behavior of a wave and how diffraction and interference reveal the wave nature of light

State Huygens's principle and use it to explain how a wavefront bends (diffracts) at an opening or edge

Describe Young's double-slit experiment and explain how constructive and destructive interference produce bright and dark fringes

Calculate an unknown quantity (wavelength, slit separation, slit width, or angle) using the double-slit conditions d sinθ = mλ and d sinθ = (m + ½)λ

Use the single-slit minimum conditions D sinθ = mλ and Dy/L = λ to relate slit width, angle, and wavelength

17.1

"As is true for all waves, light travels in straight lines and acts like a ray when it interacts with objects several times as large as its wavelength. However, when it interacts with smaller objects, it displays its wave characteristics prominently. Interference is the identifying behavior of a wave." "Huygens's principle states, 'Every point on a wavefront is a source of wavelets that spread out in the forward direction at the same speed as the wave itself. The new wavefront is a line tangent to all of the wavelets.'" "The bending of a wave around the edges of an opening or an obstacle is called diffraction. Diffraction is a wave characteristic that occurs for all types of waves." "To obtain constructive interference for a double slit, the path-length difference must be an integral multiple of the wavelength, or $d \sin\theta = m\lambda$, for $m = 0, 1, -1, 2, -2, \ldots$ (constructive)." "Similarly, to obtain destructive interference for a double slit, the path-length difference must be a half-integral multiple of the wavelength, or $d \sin\theta = (m + \tfrac{1}{2})\lambda$."