![]() ![]() Where the crest of one wave overlaps with the crest of another wave, the two waves combine to make a bigger wave, and you see a bright blob of light. The light waves that go through the slit spread out, overlap, and add together, producing the diffraction pattern you see. The black bands between the blobs of light show that a wave is associated with the light. Rotate each object while you look through it. Look at the light through a piece of cloth, a feather, a diffraction grating, or a piece of metal screen. Rotate the hair and watch the line of blobs rotate. Move the hair until it is between your eye and the light source, and notice that the light is spread into a line of blobs by the hair, just as it was by the slit. Stretch a hair tight and hold it about 1 inch (2.5 cm) from your eye. Notice that the blobs have blue and red edges and that the blue edges are closer to the light source. As you squeeze the slit together, the blobs of light grow larger and spread apart, moving away from the central light source and becoming easier to see. If you look closely you may see that the line is composed of tiny blobs of light. While looking through the slit, rotate the pencils until they are horizontal, and notice that the line of light becomes vertical. Notice that there is a line of light perpendicular to the slit. Squeeze the pencils together, making the slit smaller. Hold both pencils close to one eye (about 1 inch away) and look at the light source through the slit between the pencils. The tape wrapped around one pencil should keep the pencils slightly apart, forming a thin slit between them, just below the tape. Hold up the two pencils, side by side, with the erasers at the top. In 1907 the British physicist Ernest Rutherford showed that alpha particles are the nuclei of helium atoms.Place the light on a stable surface at least one arm’s length away from you. The gas was first isolated from terrestrial sources in 1895 by the British chemist William Ramsay, who discovered it in cleveite, a uranium-bearing mineral. Shortly afterward it was identified as an element and named by the British chemist Edward Frankland and the British astronomer Joseph Norman Lockyer. The French astronomer Pierre Janssen (1824-1907) discovered helium in the spectrum of the corona of the Sun during an eclipse in 1868.Norman Lockyear in the spectrum of a solar eclipse. Two German scientists, Robert Bunsen (a chemist) and Gustav Kirchhoff (a physicist), showed that the dark lines in the spectrum of sunlight corresponded to the bright lines of excited elements seen in laboratories on the Earth.These spectral lines are wider since they came from a wider source. The discrete spectrum of a fluorescent light. The discrete spectrum of excited helium gas The discrete spectrum of excited mercury vapor Laser light through a diffraction grating.Ĭontinuous: temperature from Wien's Displacement Law Multiple source interference (diffraction grating) ![]() single source interference (self interference) Laser light through two closely spaced slits. animate at your own riskĭouble source interference. Do not click on the animations if you have photosensitive epilepsy or a similar condition. The sources are closer together in the image on the right. (The interference fringes will be dealt with later in this section.) This image sequence is also available as an animated gif. Note that as the opening gets narrower, the amount of diffraction in the horizontal direction increases. Why is it possible to hear around corners, but not see around them? Diffraction of laser light through a vertical opening of decreasing width. Diffraction is the bending or spreading of light through an opening or around an obstacle. ![]()
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