The Wave Nature of Light Chapter 24

Properties of Light Properties of light include reflection, refraction, interference, diffraction, and dispersion. Ray diagrams model reflection and refraction (geometrical optics). Other properties can be understood by modeling light as a wave (wave optics). Note that properties of light can equivalently be modeled as a particle! This model involves quantum mechanics.

Properties of Light Geometrical optics describe how images can be formed using mirrors and lenses. Wave optics describe the colored patterns in a soap bubble, an oil slick, or on a CD.

Coherent Light Coherent light is light consisting of waves that are in phase with one another. Most light is not coherent (nor monochromatic)

Interference with Water Waves Notice bands of light and dark regions

Young’s Double Slit Experiment Interference with light is not easily observed because it has relatively short wavelengths and is not generally coherent or monochromatic. To produce two sources of coherent light, Young passed light first through a single slit to produce coherent light, then through two slits to produce two sources of coherent light.

Young’s Double Slit Experiment

Young’s Double Slit Experiment Light waves incident on two slits form spherical waves. Where crests line up with crests, constructive interference occurs. Where crests line up with troughs, destructive interference occurs.

Young’s Double Slit Experiment

Math! For Constructive Interference, ΔL = nλ From the geometry, ΔL=dsinθ Substitute: for small angles θ, sinθ = tanθ = y/L So y = nLλ/d gives the location of the interference maxima for n = 0, 1, 2, 3…

Example Monochromatic light passes through two narrow slits that are 0.050 mm apart. The interference pattern is observed on a white wall 1.0 meter from the slits, and the second order maximum is at an angle of 1.5°. a) if the slit separation decreases, what happens to the angle? b) What is the wavelength of the light and what is the distance between the n=2 and n=3 interference fringes? c) if d = 0.040 mm, what is θ2?

Diffraction What would you see with Young’s Double Slit experiment if waves weren’t diffracted at the openings? i.e. If waves always traveled in straight lines like rays? Waves don’t always travel in straight lines… they spread out as they pass through the slits. They diffract.

Diffraction Diffraction is the spreading of a wave at an opening or around an edge or corner. Generally, the longer the wavelength compared with the width of the opening, the greater the diffraction.

Single Slit Diffraction The condition for relative minima is: wsinθ = mλ m = 1, 2, 3 Using sinθ = y/L y = m(Lλ/w)

Single Slit Diffraction

Width of Central Maximum