![]() ![]() The fringes are formed due to interference of light from the two slits: The intensity of the fringes in the centre of this band is virtually uniform. In practical experiments we only use the fringes that are in the central maximum of the single slit envelope. The fringes are evenly spaced and parallel to the double slits. Using a laser means you can dispense with the single slit altogether and have a brighter coherent source and you can make 'D' really long.Īs a result of this experiment alternate bright and dark fringes, referred to as Young's fringes,can be seen on a white screen placed where the diffracted light from the double slits overlaps and produces an interference pattern. You would need to have the blinds pulled down and be limited as to how long you could make 'D'. The intensity of the light allowed throug would be too low to perform this experiment in a well lit room. In the above arrangement the single slit improves the spatial coherence of the light from the source - it is already improved in temporal coherence by being monochromatic. Double slits - to produce Young's Slits pattern of fringes You therefore need to aim to have the slit narrow enough to produce equally spaced ones but not so narrow that the overall intensity is too low. If the single slit is too wide, the dark fringes of the double slit pattern become narrower than the bright fringes, and contrast is lost between the dark and the bright fringes. Then both slits are hit by waves from this single point source. The waves from a single point source would be spacially coherent, so the single slit creates a point source in the arrangement. The purpose of the single slit is to make the source of waves that hit the double slit arrangement as coherent as possible. Single slit - needed for all light sources other than the laser Or we could use a laser - that is totally coherent - no filter and no single slit would be needed if the aperture of the laser was small enough. (A single slit would still be needed though, to make two spacially coherent sources from the monochromatic source. It would be better to just have a monochromatic source of light.Ī sodium lamp gives out yellow light of two wavelengths that are very close to each other – virtually monochromatic - temporally coherent. Nowadays we can use a bulb instead of a candle – for safety reasons! But in order to get monochromatic light from it we would have to use a filter like Young and filters absorb a lot of the light energy. In Young's day he performed the experiment using a candle flame! If we illuminate two closely spaced parallel slits (double slits) using a suitable light source the two slits act as two coherent sources of light waves, meaning the slits emit light waves with a constant phase difference and the same frequency. To observe interference of light, we need a coherent source. A simple experiment to observe Young's Fringes In addition to his observations on light interference, Young postulated that light of different colors was composed of waves having different lengths, a fundamental concept that is widely accepted today.(In contrast, the particle theory advocates envisioned that various colors were derived from particles having either different masses or traveling at different speeds). Young's conclusions were not widely accepted at the time, primarily because of the overwhelming belief in the particle theory. After being diffracted, the light that is recombined by interference the constructive and destructive interference produced a series of bright and dark fringes along the length of a screen. These 'in step waves' were then directed onto a double slit - effectively producing two coherent light sources. Diffracting light through a slingle slit he produced coherent waves. Young reasoned that if light was made of waves some type of interaction would occur when two light waves met. Now that couldn't be explained by particles! ![]() In 1803, the corpuscular theory was widely held as being true but an English physicist named Thomas Young demonstrated the interference of light. The nature of light had been argued about for centuries. A Level and AS level - UK KS 5 (Age 16 - 18).GCSE and 'O' Level - UK KS4 (Age 14 - 16). ![]()
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