6D10.34 - Coherence Length - White Light vs. Laser
To demonstrate the difference in coherence length of white light vs. laser light using a double slit interference pattern.
Pull out the optical bench with the mounted color camera (purple body), double slit, and microscope slide. Pull out the special light sources (mini-mag lite and attenuated laser on single mount). Place the light sources at least 5 feet away from the camera setup. Make sure the double slit image of both lights are visible in the camera output. Check that the microscope slide is vertical and that the demo is working properly.
Make sure the camera output is visible to the audience. Turn on the laser and the white light source. Make sure the image of the double slit interference pattern for both light sources is visible on the camera output. Carefully move the enclosed microscope slide over one of the slits of the double slit card. You should observe no change in the laser light image, but you should see the white light image go from being a double slit interference pattern to a single slit interference pattern when one slit is covered. If you continue to move the microscope slide until it covers both slits, then the double slit interference pattern will appear again.
When light from a point source strikes a double slit, each slit produces a single-slit diffraction pattern superimposed on one other. When the light is coherent, the two waves interfere and generate the Young double slit pattern. When the two light waves are not coherent, one observes the single slit diffraction pattern. Covering one slit adds an additional (1-n)*(t/λ) wavelengths of light to its path between the point light source and your eye, relative to the number of wavelengths along the path that goes through the uncovered slit to your eye. Here, n is the index of refraction of glass, λ is the wavelength of light, and t is the thickness of the glass plate. With n=1.5, λ ≈ 550nm, and t ≈ 1mm, this is an additional ≈ 880 wavelengths. Since the coherence length of white light is much shorter than this (only a few wavelengths), the waves are not coherent at your eye and the interference pattern is not seen.