![]() ![]() The narrower the slit, the more the light spreads out. ![]() This different amount of bending gives the blobs their colored edges: blue on the inside, red on the outside. Red light, for instance, has a longer wavelength than blue light, so it bends more than blue light does. The angle at which the light bends is proportional to the wavelength of the light. Where the trough of one wave overlaps with the crest of another wave, the waves cancel each other out, and you see a dark band. 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. Numerical computations are carried out in full, giving the vector pressure ratio at the pole facing the source for spheres of various diameters and at various frequencies throughout the acoustic range.Place the light on a stable surface at least one arm’s length away from you. Theory of the diffraction of a sound wave by a rigid sphere.-The theory of the diffraction of a plane wave of the type exp i ω ( t − x V ) by a rigid sphere is outlined in terms of Hankel's H 2 n + 1 2 functions, for which tables exist up to the highest orders required for the computations in practical cases. It is proposed to evaluate the correction for diffraction by employing a standard spherical mounting of which the diaphragm occupies a small area about the pole the increase in pressure for this mounting can be calculated theoretically, and the correction for other mountings can then be obtained by experimental comparison. Because of the mathematically irregular shape of the conventional microphone and its mounting the effect cannot be calculated. Proposed method of evaluating the pressure correction made necessary by diffraction.-The diffraction of sound around the diaphragm of the microphone ordinarily used in the measurement of the instantaneous pressure in a sound wave causes the indicated pressure to vary from equality with the actual pressure in the undisturbed wave at low frequencies, to twice this pressure at high frequencies.
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