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4.1 Young's Double Slit Experiment Equipment Optical Bench Double slit slide Optical Mounts White light source LED light sources Adjustable slit Telescope Activity The double slit slide has several sets of slits. Use black tape to leave one set of slits only. Set up the white light or white LED source on the optical bench. Bring the slit up close to the eye and view the light source. What do you see? The interference pattern can only occur when the light diffracted by the two slits is coherent or in phase with each other. Coherence can be achieved with a laser, however Thomas Young performed this experiment in 1801 and did not have access to such a device. Instead Young used the arrangement shown in Figure 4.1 in which single and double slits are employed to produce two coherent light sources. The light forms a spectrum from each slit and constructive or destructive interference occurs depending on the phase difference of the two paths for the light. Figure 4.1 Young's double slit experiment Acton Instruments - ANU Page 1 4/30/2017 Young's experiment provided strong evidence for the wave nature of light and the measurement of wavelength. The formation of spectra and interference effects are exploited in the design and use of diffraction gratings. The interference patterns or fringes can be investigated in more detail using a single slit in front of the light source and the telescope to view the fringes produced by the double slit. Refer to figure 4.2 telescope double slit single slit LED Figure 4.2 Observing double slit interference fringes The single slit fits into one of the magnetic holders horizontally or vertically and can be held open using the screws. Try different double slits but block out the remainder. Adjust the components and alignment to produce the best fringe pattern possible. Use the different coloured LEDs. Sketch the fringe patterns and note any differences. Explain the interference patterns and results in terms of the wave like nature of light. How are the colours and separation of fringes related to the wavelength of the light? Acton Instruments - ANU Page 2 4/30/2017